Dissociative recombination in ultraviolet filamentary plasma gratings Hui Zhou, Wenxue Li,* Di Wang, Liping Shi, Liang’en Ding, and Heping Zeng State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200062, China * [email protected]
Abstract: We investigated collisions of nitrogen and argon gas mixture with energetic electrons accelerated by Bragg incident intense infrared femtosecond laser pulses in ultraviolet filamentary plasma gratings. Significant decrease of fluorescence spectra of argon atoms were observed when a small amount of nitrogen gas was mixed with argon gas that facilitated observable argon-nitrogen collisions. We experimentally measured the fluorescence emission from the argon and nitrogen gas mixture under different driving pulse energies, the fluorescence decay dynamics after the impact excitation, as well as the fluorescence intensity dependence on the nitrogen and argon pressures. The experimental measurements were based on the electron acceleration and its subsequent impact with the gas mixture in the filamentary plasma gratings, which was essential for the observation of the dominant dissociative recombination in the gas mixture. ©2014 Optical Society of America OCIS codes: (320.7110) Ultrafast nonlinear optics; (320.7150) Ultrafast spectroscopy; (140.3610) Lasers, ultraviolet; (020.2649) Strong field laser physics.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
P. L. Kelley, “Self-focusing of optical beams,” Phys. Rev. Lett. 15(26), 1005–1008 (1965). S. Tzortzakis, G. Méchain, G. Patalano, Y.-B. André, B. Prade, M. Franco, A. Mysyrowicz, J.-M. Munier, M. Gheudin, G. Beaudin, and P. Encrenaz, “Coherent subterahertz radiation from femtosecond infrared filaments in air,” Opt. Lett. 27(21), 1944–1946 (2002). B. Prade, M. Franco, A. Mysyrowicz, A. Couairon, H. Buersing, B. Eberle, M. Krenz, D. Seiffer, and O. Vasseur, “Spatial mode cleaning by femtosecond filamentation in air,” Opt. Lett. 31(17), 2601–2603 (2006). A. Couairon, J. Biegert, C. P. Hauri, W. Kornelis, F. W. Helbing, U. Keller, and A. Mysyrowicz, “Self-compression of ultra-short laser pulses down to one optical cycle by filamentation,” J. Mod. Opt. 53, 75–85 (2006). A. G. York, H. M. Milchberg, J. P. Palastro, and T. M. Antonsen, “Direct acceleration of electrons in a corrugated plasma waveguide,” Phys. Rev. Lett. 100(19), 195001 (2008). X. Yang, J. Wu, Y. Peng, Y. Tong, P. Lu, L. Ding, Z. Xu, and H. Zeng, “Plasma waveguide array induced by filament interaction,” Opt. Lett. 34(24), 3806–3808 (2009). Y. Wang, Y. Zhang, P. Chen, L. Shi, X. Lu, J. Wu, L. Ding, and H. Zeng, “The formation of an intense filament controlled by interference of ultraviolet femtosecond pulses,” Appl. Phys. Lett. 98(11), 111103 (2011). Y. Liu, M. Durand, S. Chen, A. Houard, B. Prade, B. Forestier, and A. Mysyrowicz, “Energy exchange between femtosecond laser filaments in air,” Phys. Rev. Lett. 105(5), 055003 (2010). C.-C. Kuo, C.-H. Pai, M.-W. Lin, K.-H. Lee, J.-Y. Lin, J. Wang, and S.-Y. Chen, “Enhancement of relativistic harmonic generation by an optically preformed periodic plasma waveguide,” Phys. Rev. Lett. 98(3), 033901 (2007). X. Yang, J. Wu, Y. Peng, Y. Tong, S. Yuan, L. Ding, Z. Xu, and H. Zeng, “Noncollinear interaction of femtosecond filaments with enhanced third harmonic generation in air,” Appl. Phys. Lett. 95(11), 111103 (2009). X. Yang, J. Wu, Y. Peng, S. Yuan, and H. Zeng, “Experimental observation of noncollinear coupling of filaments in air,” in Conference on Lasers and Electro-Optics/Quantum Electronics and Laser Science, Technical Digest (Optical Society of America, 2008), paper CTuE6. H. Wu, T. Yang, Y. Wang, and L. Ding, “Background-free third-order harmonic generation induced by dynamic gratings in dual filaments,” J. Opt. Soc. Am. B 26(4), 645–649 (2009). X. Yang, J. Wu, Y. Tong, L. Ding, Z. Xu, and H. Zeng, “Femtosecond laser pulse energy transfer induced by plasma grating due to filament interaction in air,” Appl. Phys. Lett. 97(7), 071108 (2010). S. Suntsov, D. Abdollahpour, D. G. Papazoglou, and S. Tzortzakis, “Femtosecond laser induced plasma diffraction gratings in air as photonic devices for high intensity laser applications,” Appl. Phys. Lett. 94(25), 251104 (2009).
#205168 - $15.00 USD (C) 2014 OSA
Received 21 Jan 2014; revised 24 Mar 2014; accepted 23 Apr 2014; published 30 Apr 2014 5 May 2014 | Vol. 22, No. 9 | DOI:10.1364/OE.22.010968 | OPTICS EXPRESS 10968
15. L. Shi, W. Li, H. Zhou, L. Ding, and H. Zeng, “Impact excitation of neon atoms by heated seed electrons in filamentary plasma gratings,” Opt. Lett. 38(4), 398–400 (2013). 16. L. Shi, W. Li, D. Bai, H. Zhou, D. Wang, L. Ding, and H. Zeng, “Enhanced fluorescence emission of helium atoms by seeded optically driven impact excitation,” Phys. Rev. A 88(1), 013418 (2013). 17. L. Shi, W. Li, Y. Wang, X. Lu, L. Ding, and H. Zeng, “Generation of high-density electrons based on plasma grating induced Bragg diffraction in air,” Phys. Rev. Lett. 107(9), 095004 (2011). 18. A. Couairon and A. Mysyrowicz, “Femtosecond filamentation in transparent media,” Phys. Rep. 441(2-4), 47–189 (2007). 19. M. N. Rolin, S. I. Shabunya, J. C. Rostaing, and J. M. Perrin, “Self-consistent modelling of a microwave discharge in neon and argon at atmospheric pressure,” Plasma Sources Sci. Technol. 16(3), 480–491 (2007). 20. Y. Itikawa, “Cross sections for electron collisions with nitrogen molecules,” J. Phys. Chem. Ref. Data 35(1), 31–54 (2006). 21. V. Puech and L. Torchin, “Collision cross sections and electron swarm parameters in argon,” J. Phys. D Appl. Phys. 19(12), 2309–2323 (1986). 22. M. N. Shneider, A. Baltuska, and A. M. Zheltikov, “Population inversion of molecular nitrogen in an Ar: N2 mixture by selective resonance-enhanced multiphoton ionization,” J. Appl. Phys. 110(8), 083112 (2011). 23. A. Filin, R. Compton, D. A. Romanov, and R. J. Levis, “Impact-ionization cooling in laser-induced plasma filaments,” Phys. Rev. Lett. 102(15), 155004 (2009). 24. J. Liu and X. C. Zhang, “Terahertz-radiation-enhanced emission of fluorescence from gas plasma,” Phys. Rev. Lett. 103(23), 235002 (2009). 25. P. P. Kiran, S. Bagchi, S. R. Krishnan, C. L. Arnold, G. R. Kumar, and A. Couairon, “Focal dynamics of multiple filaments: microscopic imaging and reconstruction,” Phys. Rev. A 82(1), 013805 (2010).
1. Introduction Ultrafast laser filaments [1] arisen from dynamic balance between Kerr self-focusing and multiphoton-ionization induced plasma defocusing have attracted much attention in recent years and stimulated promising applications in terahertz generation [2], spatial self-cleaning [3], pulse compression [4], and so on. Especially, nonlinear interaction of filaments is burgeoning various spectroscopic applications in plasma channels, which has already been used to control high-field direct acceleration of electrons over many centimeters [5], dramatic increase of plasma fluorescence in the interaction region [6,7], the energy exchange between femtosecond laser filaments [8], and the phase-matched enhancement of relativistic third harmonic (TH) generation [9,10]. Noncollinear interaction of synchronized filaments was demonstrated to form filamentary plasma gratings [11–14], benefiting observable impact ionization and electron-impact excitation of atoms and molecules [15,16]. In plasma gratings, buffer gas with low ionization potential could liberate seed electrons through multi-photon ionization, a subsequent intense infrared filament could drive the liberated electrons to gain high kinetic energies, and then electron-atom collision populates a substantial amount of neutral atoms of high ionization potential into high-lying excited states, resulting in significant intensity enhancement of fluorescence emission from gas mixture of different atoms or molecules. It is interesting to note that the liberated electrons experience quite high peak intensity in the plasma gratings [6,8]. This impact excitation and ionization thus may be solved as a useful method to excite atoms or molecules of high ionization potentials. If the gas mixture containing atoms or molecule of almost the same ionization potentials, the atomic or molecular collisions may become important, and physical mechanisms for the impact excitation or ionization could be quite different, which awaits further experimental exploration. In this paper, we address this interesting issue with prototype nitrogen and argon gas mixture. Filamentary plasma gratings were formed by using two crossly overlapped ultraviolet femtosecond pulses and intense infrared femtosecond pulses were coupled into the preformed plasma gratings to heat the liberated electrons from argon atoms through multiphoton absorption of ultraviolet photons. Atomic collisions with energetic electrons populated neutral argon atoms in high-lying excited states. A mixture with a small amount of nitrogen gas was demonstrated to significantly decrease the argon atomic and ionic fluorescence, suggesting that dissociative recombination played a dominant role in the argon and nitrogen gas mixture, and a substantial amount of nitrogen atoms was populated into high-lying excited states through dissociative recombination and collisions between argon atoms and nitrogen molecules. The
#205168 - $15.00 USD (C) 2014 OSA
Received 21 Jan 2014; revised 24 Mar 2014; accepted 23 Apr 2014; published 30 Apr 2014 5 May 2014 | Vol. 22, No. 9 | DOI:10.1364/OE.22.010968 | OPTICS EXPRESS 10969
use of preformed plasma grating and the heating infrared intense laser pulses was essential for the observation of characteristic fluorescence intensity and its dynamic evolution. 2. Experimental results The experiment was schematically illustrated in Fig. 1. Intense third-harmonic (TH, 267 nm) pulses with the pulse energy up to 1.5 mJ and pulse duration ~100 fs were generated from an 800-nm Ti-sapphire regenerative laser of 40 fs pulse duration and 25 mJ pulse energy. The generated TH pulses were split into two beams and non-collinearly focused into a sealed chamber with a convex lens ( f =10 cm ). Gas composition and pressure was well controlled in the chamber. A high-electron-density plasma grating with periodically modulated refractive index was generated as the TH filaments were synchronized in the overlapped region [15, 16]. There remained about 8 mJ fundamental-wave (FW) pulses after the TH generation, which was focused onto the plasma grating at its 1st Bragg diffraction angle (~4°) with a convex lens of 10 cm focal length. Such a Bragg incident intense FW pulses in the plasma gratings benefited electron acceleration in the well-confined periodic plasma channels. Thus, numerous atoms, molecules and ions inside the chamber were kicked into highly excited states by electron collision in avalanche-like processes [17], giving rising to a significant enhancement of fluorescence intensity (FI).
Fig. 1. Schematic experimental setup for electron-atom collision in ultraviolet filamentary plasma gratings established by two noncollinearly crossed TH filaments. Intense FW pulses were incident into the plasma gratings at the 1st Bragg angle. The gas mixture was controlled with various argon and nitrogen gas pressures.
The fluorescence spectrum of Ar atoms shown in Fig. 2 was integrated by 20 pulses to reduce the error caused by pulse-to-pulse energy fluctuation. The lower black curve indicated the fluorescence spectrum from two TH filamentary plasma grating in pure Ar gas with the pressure of 0.5 atm. Only a few of neutral atomic lines were observed. With FW filament injected into the plasma gratings, these preexisted atomic lines increased dramatically. Furthermore, a host of additional ionic Ar lines appeared on a continuum background resulting from the Bremsstrahlung emission, as shown by the middle blue curve. This could be ascribed to heating of liberated electrons driven by the infrared filament inside the plasma channels. Electrons were liberated from Ar atoms through multiphoton absorption of ultraviolet photons due to its relatively low ionization (15.759 eV), which underwent electron acceleration by the subsequent FW laser electric field, and acquired kinetic energy scaled as I λ 2 . Accordingly, the electron density grew exponentially after the passage of the FW pulses as ne (td , t ) = ne (td , 0) exp(σ It / U i ) , where the initial seed-electron density ne (td , 0) and the inverse Bremsstrahlung cross section σ are both proportional to gas pressure P [18], I and t denote the intensity and duration of the heating FW pulse. Finally, numerous neutral argon
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Received 21 Jan 2014; revised 24 Mar 2014; accepted 23 Apr 2014; published 30 Apr 2014 5 May 2014 | Vol. 22, No. 9 | DOI:10.1364/OE.22.010968 | OPTICS EXPRESS 10970
atoms were subsequently kicked into the ionic states as a result of atomic collisions with these energetic electrons Ar+e → Ar + +e+e.
(1)
More high-lying excited states were populated through ion conversion accompanied by dissociative recombination [19] Ar+Ar + +Ar → Ar2 + +Ar; Ar2 + +e → Ar+Ar* ,
(2)
through which Ar atoms were populated in the excited state 4p (Ar*).The electron ion collision could also excite the Ar ion to the highly excited state through the process as Ar2 + +e → (Ar + )* +Ar+e.
(3)
This gives rising to Ar ionic fluorescence with the emission intensity proportional to the square of the gas pressure.
Fig. 2. Measured fluorescence spectra from TH plasma gratings in 0.5 atm Ar (lower black curve), from TH plasma gratings with Bragg incident FW pulses in 0.5 atm Ar (middle blue curve) and in 0.5 atm Ar mixed with 0.5 atm N2 (upper red curve).
When subsequently injecting N2 gas into the chamber, significant decrease of Ar atomic fluorescence was observed, as shown in Fig. 2 (the upper red curve). In order to reveal the mechanism of this observation, we studied the dependence of the continuum and argon atomic fluorescence intensity on the N2 pressure. As shown in Fig. 3(a) (red circle), the FI of Ar atomic characteristic line centered at 706.7 nm decreased rapidly when the first 0.1 atm N2 gas was injected in the chamber. All the characteristic lines from 600 to 1020 nm arisen from the 4p→4s transitions of the neutral Ar atom exhibited a similar decline with the FI decrease factor varied from 5 to 9. Further increase of N2 gas pressure above 0.1 atm, the FI only showed a slight decline. The distinct decline difference at different N2 gas pressure hinted competition between different collision mechanisms. As the inverse Bremsstrahlung cross section σ of electron-Ar is larger than that of electron-N2, ( σ = 2.94 × 10−19 cm 2 for Ar and σ = 2.05 ×10−19 cm 2 for N2) [20, 21], only a small amount of N2 injection (with about one quarter of Ar gas pressure in our experiment) was sufficient to saturate the rapid decline of the Ar atomic excitation by
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Received 21 Jan 2014; revised 24 Mar 2014; accepted 23 Apr 2014; published 30 Apr 2014 5 May 2014 | Vol. 22, No. 9 | DOI:10.1364/OE.22.010968 | OPTICS EXPRESS 10971
electron-argon impact. Further increase of the N2 gas pressure may influence Ar atomic impact excitation through collisions of Ar and N2.
Fig. 3. (a) Measured intensity of integrated continuum background (blue squares) and argon fluorescence spectra at 706.7 nm (red circles) from the mixture gases as a function of nitrogen gas pressure with argon gas fixed at 0.5 atm. Inset, the estimated electron density as a function of nitrogen gas pressure with argon gas fixed at 0.5 atm. (b) Measured intensity of ionic argon fluorescence spectra at 358.8 nm (red circles), atomic nitrogen spectra at 904.9 nm (blue squares) and ionic nitrogen spectra at 500.5 nm (olive triangles).
The FI decrease of atomic argon (Ar*) suggested that dissociative recombination played a dominant role with a small amount of nitrogen in the Ar-N2 gas mixture [22], Ar* +N 2 → Ar+N+N* ,
(4)
Ar + +N 2 → Ar+N 2 + ; N 2 + +e → N+N* .
(5)
The above reactions hinted that there should be two mechanisms in the dissociation of N2 inside the mixture of N2 and Ar gas. The first 0.1 atm N2 was apt to combine with the existed Ar* and Ar+ instead of directly dissociated by the laser beam. The decline of continuum background intensity as shown in Fig. 3(a) (blue squares) revealed the decrease of electron density [23], confirming the existence of the reaction (4) and (5). The estimated electron density was shown in the inset of Fig. 3(a). The initial electron density was about 1.06 × 1019 cm−3 when the cell was filled with 0.5 atm Ar, and after the injection of 0.1 atm N2, the electron density decreased to 0.9 × 1019 cm−3. However, when the pressure of N2 gas continued to increase, multi-photon absorption took advantage in this process which brought continuous increasing of the electron density. The increase of integrated continuum background intensity with more than 0.1 atm N2, exhibited in Fig. 3(a) (blue squares), was ascribed to the high-density electrons which were liberated from N2. Meanwhile, the recombination between N2 and Ar* or Ar+ always existed which could be clearly seen from Fig. 3(b) (red circles), the FI of Ar+, we take 358.8 nm (4d→4p) for example, kept to decrease with the increase of N2 pressure. The nitrogen atoms were also kicked into ionic states which promoted the avalanche ionization of neutral atoms and exponential increase of free-electron density: N+e → (N + )* +e+e.
(6)
With numerous N+ by energetic electron collision, a substantial amount of nitrogen atoms was populated into high-lying excited states through ion conversion and dissociative recombination processes N+N + +N → N 2 + +N; N 2 + +e → N+N* .
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(7)
Received 21 Jan 2014; revised 24 Mar 2014; accepted 23 Apr 2014; published 30 Apr 2014 5 May 2014 | Vol. 22, No. 9 | DOI:10.1364/OE.22.010968 | OPTICS EXPRESS 10972
The intensity of nitrogen atomic fluorescence (904.9 nm, 3p→3s, blue squares) and ionic fluorescence (500.5 nm, 3d→3p, olive triangles) increased with the nitrogen pressure. As shown in Fig. 3(b), the measured intensity of nitrogen atomic fluorescence exhibited a nitrogen gas pressure dependence scaling as P , suggesting the observed FI was proportional to the total number of nitrogen atoms. This further confirmed the dissociative recombination processes (6) and (7). We further investigated the dependence of fluorescence intensity on argon gas pressure while fixed the nitrogen pressure at 0.5 atm, as shown in Fig. 4. Although the FI of argon increased with its pressure (red square and blue triangle), as shown in Fig. 4(a), it was significantly weaker in comparison with the FI of pure argon at the same pressure, confirming the existence of reaction (4) and (5). Figure 4(b) shows that the nitrogen atomic fluorescence at 904.9 nm (blue square) and nitrogen ionic fluorescence at 500.5 nm (red circle) increased rapidly, which could be understood from electron-atom collision induced ionization [24] since the electron density increased rapidly [black circle in Fig. 4(a)]. The Ar atomic and ionic fluorescence intensity exhibited an Ar gas pressure dependence of P 2 , confirming the dissociative recombination processes (2) and (3). The linear dependence of nitrogen FI upon the Ar gas pressure as shown in Fig. 4(b) was consistent with the processes (4-6).
Fig. 4. Measured intensity of spectra emission from the gas mixture of 0.5 atm fixed nitrogen gas pressure and various argon gas pressure: (a) integrated continuum background (black circles), argon atomic fluorescence spectra at 706.7 nm (blue triangles), argon ionic fluorescence spectra at 358.8 nm (red squares); (b) nitrogen atomic fluorescence spectra at 904.9 nm (blue squares) and nitrogen ionic fluorescence spectra at 500.5 nm (red circles).
The characteristic fluorescence intensity of the gas mixture was also experimentally investigated under irradiation of FW laser pulses with different pulse energies, as shown in Fig. 5(a). Here, we chose ionic nitrogen fluorescence spectra at 500.5 nm as an example. The blue circles show the nitrogen fluorescence intensity versus the FW pulse energy, with its nonlinear fit according to F ∝ I × e I [24], where I denotes laser intensity. The black squares represent the measured intensity of the continuum background, which was proportional to the free electron density ( ne ), and could be well described by ne ∝ e I [25]. We next studied the FI dependence on time delay td . The fluorescence spectra from the argon and nitrogen gas mixture were measured at the same FW laser pulse energy (8 mJ) and gas pressure ratio (0.5 atm: 0.5 atm) as the time delay between the FW and TH pulses was tuned, as shown in Fig. 5(b). Here we also measured the nitrogen characteristic line at 500.5 nm for example. When the incident FW pulse arrived before TH beams ( td < 0 ), almost no fluorescence spectra were observed, indicating that the use of preformed plasma gratings to
#205168 - $15.00 USD (C) 2014 OSA
Received 21 Jan 2014; revised 24 Mar 2014; accepted 23 Apr 2014; published 30 Apr 2014 5 May 2014 | Vol. 22, No. 9 | DOI:10.1364/OE.22.010968 | OPTICS EXPRESS 10973
guide the infrared intense laser pulses was essential for the seeded electron acceleration and impact excitation of neutral atoms. The enhanced fluorescence was observed only when the FW and TH pulses were temporally overlapped, which was confirmed by the spike sharp of fluorescence intensity at td ~0 . This could be attributed to two-color fields induced ionization enhancement. The fluorescence intensity exhibited a slow monotonic decay for td longer than 2.3 ps. This should be resulted from the decrease of seed electron density induced by electron-ion recombination. The gradual decrease could be well described by [1 ne (2.3, t) + β (td − 2.3)]−1 , as fitted in Fig. 5(b), with time t and td scaling in picosecond unit, the plasma decay constant β ≈ 0.043 ps −1 , where ne (2.3, t) denotes the electron density after the passage of the FW pulse with a time delay of 2.3 ps. Such a delay was sufficient for free expansion of seed plasma [16].
Fig. 5. (a) FW pulse energy dependence of the background continuum intensity (black squares) and nitrogen fluorescence spectra at 500.5 nm (blue circles). (b) Experimentally measured dependence of nitrogen fluorescence intensity on the time delay between the FW and plasma-grating-forming TH laser pulses in the nitrogen and argon gas mixture.
3. Conclusion
In conclusion, we demonstrated dissociative recombination of argon and nitrogen gas mixture in filamentary plasma gratings. As argon atom and nitrogen molecule have comparable ionization potentials, electrons liberated from argon atoms and nitrogen molecule played almost the same role in the gas mixture. With intense infrared femtosecond pulses coupled into the preformed plasma gratings to heat the liberated electrons, electron-atom and atom-molecule collisions assisted dominant dissociative recombination that excited argon and nitrogen atoms into high-lying excited states. This may stimulate promising spectroscopic applications in the remote monitoring of atmospheric conditions. Acknowledgments
We acknowledge financial supports from the National Natural Science Fund (11274115 and 10990101), National Key Project for Basic Research (2011CB808105), National Key Scientific Instrument Project (2012YQ150092), and Innovation Program of Shanghai Municipal Education Commission (2014Z10269011).
#205168 - $15.00 USD (C) 2014 OSA
Received 21 Jan 2014; revised 24 Mar 2014; accepted 23 Apr 2014; published 30 Apr 2014 5 May 2014 | Vol. 22, No. 9 | DOI:10.1364/OE.22.010968 | OPTICS EXPRESS 10974
Abstract: We investigated collisions of nitrogen and argon gas mixture with energetic electrons accelerated by Bragg incident intense infrared femtosecond laser pulses in ultraviolet filamentary plasma gratings. Significant decrease of fluorescence spectra of argon atoms were observed when a small amount of nitrogen gas was mixed with argon gas that facilitated observable argon-nitrogen collisions. We experimentally measured the fluorescence emission from the argon and nitrogen gas mixture under different driving pulse energies, the fluorescence decay dynamics after the impact excitation, as well as the fluorescence intensity dependence on the nitrogen and argon pressures. The experimental measurements were based on the electron acceleration and its subsequent impact with the gas mixture in the filamentary plasma gratings, which was essential for the observation of the dominant dissociative recombination in the gas mixture. ©2014 Optical Society of America OCIS codes: (320.7110) Ultrafast nonlinear optics; (320.7150) Ultrafast spectroscopy; (140.3610) Lasers, ultraviolet; (020.2649) Strong field laser physics.
References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
P. L. Kelley, “Self-focusing of optical beams,” Phys. Rev. Lett. 15(26), 1005–1008 (1965). S. Tzortzakis, G. Méchain, G. Patalano, Y.-B. André, B. Prade, M. Franco, A. Mysyrowicz, J.-M. Munier, M. Gheudin, G. Beaudin, and P. Encrenaz, “Coherent subterahertz radiation from femtosecond infrared filaments in air,” Opt. Lett. 27(21), 1944–1946 (2002). B. Prade, M. Franco, A. Mysyrowicz, A. Couairon, H. Buersing, B. Eberle, M. Krenz, D. Seiffer, and O. Vasseur, “Spatial mode cleaning by femtosecond filamentation in air,” Opt. Lett. 31(17), 2601–2603 (2006). A. Couairon, J. Biegert, C. P. Hauri, W. Kornelis, F. W. Helbing, U. Keller, and A. Mysyrowicz, “Self-compression of ultra-short laser pulses down to one optical cycle by filamentation,” J. Mod. Opt. 53, 75–85 (2006). A. G. York, H. M. Milchberg, J. P. Palastro, and T. M. Antonsen, “Direct acceleration of electrons in a corrugated plasma waveguide,” Phys. Rev. Lett. 100(19), 195001 (2008). X. Yang, J. Wu, Y. Peng, Y. Tong, P. Lu, L. Ding, Z. Xu, and H. Zeng, “Plasma waveguide array induced by filament interaction,” Opt. Lett. 34(24), 3806–3808 (2009). Y. Wang, Y. Zhang, P. Chen, L. Shi, X. Lu, J. Wu, L. Ding, and H. Zeng, “The formation of an intense filament controlled by interference of ultraviolet femtosecond pulses,” Appl. Phys. Lett. 98(11), 111103 (2011). Y. Liu, M. Durand, S. Chen, A. Houard, B. Prade, B. Forestier, and A. Mysyrowicz, “Energy exchange between femtosecond laser filaments in air,” Phys. Rev. Lett. 105(5), 055003 (2010). C.-C. Kuo, C.-H. Pai, M.-W. Lin, K.-H. Lee, J.-Y. Lin, J. Wang, and S.-Y. Chen, “Enhancement of relativistic harmonic generation by an optically preformed periodic plasma waveguide,” Phys. Rev. Lett. 98(3), 033901 (2007). X. Yang, J. Wu, Y. Peng, Y. Tong, S. Yuan, L. Ding, Z. Xu, and H. Zeng, “Noncollinear interaction of femtosecond filaments with enhanced third harmonic generation in air,” Appl. Phys. Lett. 95(11), 111103 (2009). X. Yang, J. Wu, Y. Peng, S. Yuan, and H. Zeng, “Experimental observation of noncollinear coupling of filaments in air,” in Conference on Lasers and Electro-Optics/Quantum Electronics and Laser Science, Technical Digest (Optical Society of America, 2008), paper CTuE6. H. Wu, T. Yang, Y. Wang, and L. Ding, “Background-free third-order harmonic generation induced by dynamic gratings in dual filaments,” J. Opt. Soc. Am. B 26(4), 645–649 (2009). X. Yang, J. Wu, Y. Tong, L. Ding, Z. Xu, and H. Zeng, “Femtosecond laser pulse energy transfer induced by plasma grating due to filament interaction in air,” Appl. Phys. Lett. 97(7), 071108 (2010). S. Suntsov, D. Abdollahpour, D. G. Papazoglou, and S. Tzortzakis, “Femtosecond laser induced plasma diffraction gratings in air as photonic devices for high intensity laser applications,” Appl. Phys. Lett. 94(25), 251104 (2009).
#205168 - $15.00 USD (C) 2014 OSA
Received 21 Jan 2014; revised 24 Mar 2014; accepted 23 Apr 2014; published 30 Apr 2014 5 May 2014 | Vol. 22, No. 9 | DOI:10.1364/OE.22.010968 | OPTICS EXPRESS 10968
15. L. Shi, W. Li, H. Zhou, L. Ding, and H. Zeng, “Impact excitation of neon atoms by heated seed electrons in filamentary plasma gratings,” Opt. Lett. 38(4), 398–400 (2013). 16. L. Shi, W. Li, D. Bai, H. Zhou, D. Wang, L. Ding, and H. Zeng, “Enhanced fluorescence emission of helium atoms by seeded optically driven impact excitation,” Phys. Rev. A 88(1), 013418 (2013). 17. L. Shi, W. Li, Y. Wang, X. Lu, L. Ding, and H. Zeng, “Generation of high-density electrons based on plasma grating induced Bragg diffraction in air,” Phys. Rev. Lett. 107(9), 095004 (2011). 18. A. Couairon and A. Mysyrowicz, “Femtosecond filamentation in transparent media,” Phys. Rep. 441(2-4), 47–189 (2007). 19. M. N. Rolin, S. I. Shabunya, J. C. Rostaing, and J. M. Perrin, “Self-consistent modelling of a microwave discharge in neon and argon at atmospheric pressure,” Plasma Sources Sci. Technol. 16(3), 480–491 (2007). 20. Y. Itikawa, “Cross sections for electron collisions with nitrogen molecules,” J. Phys. Chem. Ref. Data 35(1), 31–54 (2006). 21. V. Puech and L. Torchin, “Collision cross sections and electron swarm parameters in argon,” J. Phys. D Appl. Phys. 19(12), 2309–2323 (1986). 22. M. N. Shneider, A. Baltuska, and A. M. Zheltikov, “Population inversion of molecular nitrogen in an Ar: N2 mixture by selective resonance-enhanced multiphoton ionization,” J. Appl. Phys. 110(8), 083112 (2011). 23. A. Filin, R. Compton, D. A. Romanov, and R. J. Levis, “Impact-ionization cooling in laser-induced plasma filaments,” Phys. Rev. Lett. 102(15), 155004 (2009). 24. J. Liu and X. C. Zhang, “Terahertz-radiation-enhanced emission of fluorescence from gas plasma,” Phys. Rev. Lett. 103(23), 235002 (2009). 25. P. P. Kiran, S. Bagchi, S. R. Krishnan, C. L. Arnold, G. R. Kumar, and A. Couairon, “Focal dynamics of multiple filaments: microscopic imaging and reconstruction,” Phys. Rev. A 82(1), 013805 (2010).
1. Introduction Ultrafast laser filaments [1] arisen from dynamic balance between Kerr self-focusing and multiphoton-ionization induced plasma defocusing have attracted much attention in recent years and stimulated promising applications in terahertz generation [2], spatial self-cleaning [3], pulse compression [4], and so on. Especially, nonlinear interaction of filaments is burgeoning various spectroscopic applications in plasma channels, which has already been used to control high-field direct acceleration of electrons over many centimeters [5], dramatic increase of plasma fluorescence in the interaction region [6,7], the energy exchange between femtosecond laser filaments [8], and the phase-matched enhancement of relativistic third harmonic (TH) generation [9,10]. Noncollinear interaction of synchronized filaments was demonstrated to form filamentary plasma gratings [11–14], benefiting observable impact ionization and electron-impact excitation of atoms and molecules [15,16]. In plasma gratings, buffer gas with low ionization potential could liberate seed electrons through multi-photon ionization, a subsequent intense infrared filament could drive the liberated electrons to gain high kinetic energies, and then electron-atom collision populates a substantial amount of neutral atoms of high ionization potential into high-lying excited states, resulting in significant intensity enhancement of fluorescence emission from gas mixture of different atoms or molecules. It is interesting to note that the liberated electrons experience quite high peak intensity in the plasma gratings [6,8]. This impact excitation and ionization thus may be solved as a useful method to excite atoms or molecules of high ionization potentials. If the gas mixture containing atoms or molecule of almost the same ionization potentials, the atomic or molecular collisions may become important, and physical mechanisms for the impact excitation or ionization could be quite different, which awaits further experimental exploration. In this paper, we address this interesting issue with prototype nitrogen and argon gas mixture. Filamentary plasma gratings were formed by using two crossly overlapped ultraviolet femtosecond pulses and intense infrared femtosecond pulses were coupled into the preformed plasma gratings to heat the liberated electrons from argon atoms through multiphoton absorption of ultraviolet photons. Atomic collisions with energetic electrons populated neutral argon atoms in high-lying excited states. A mixture with a small amount of nitrogen gas was demonstrated to significantly decrease the argon atomic and ionic fluorescence, suggesting that dissociative recombination played a dominant role in the argon and nitrogen gas mixture, and a substantial amount of nitrogen atoms was populated into high-lying excited states through dissociative recombination and collisions between argon atoms and nitrogen molecules. The
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Received 21 Jan 2014; revised 24 Mar 2014; accepted 23 Apr 2014; published 30 Apr 2014 5 May 2014 | Vol. 22, No. 9 | DOI:10.1364/OE.22.010968 | OPTICS EXPRESS 10969
use of preformed plasma grating and the heating infrared intense laser pulses was essential for the observation of characteristic fluorescence intensity and its dynamic evolution. 2. Experimental results The experiment was schematically illustrated in Fig. 1. Intense third-harmonic (TH, 267 nm) pulses with the pulse energy up to 1.5 mJ and pulse duration ~100 fs were generated from an 800-nm Ti-sapphire regenerative laser of 40 fs pulse duration and 25 mJ pulse energy. The generated TH pulses were split into two beams and non-collinearly focused into a sealed chamber with a convex lens ( f =10 cm ). Gas composition and pressure was well controlled in the chamber. A high-electron-density plasma grating with periodically modulated refractive index was generated as the TH filaments were synchronized in the overlapped region [15, 16]. There remained about 8 mJ fundamental-wave (FW) pulses after the TH generation, which was focused onto the plasma grating at its 1st Bragg diffraction angle (~4°) with a convex lens of 10 cm focal length. Such a Bragg incident intense FW pulses in the plasma gratings benefited electron acceleration in the well-confined periodic plasma channels. Thus, numerous atoms, molecules and ions inside the chamber were kicked into highly excited states by electron collision in avalanche-like processes [17], giving rising to a significant enhancement of fluorescence intensity (FI).
Fig. 1. Schematic experimental setup for electron-atom collision in ultraviolet filamentary plasma gratings established by two noncollinearly crossed TH filaments. Intense FW pulses were incident into the plasma gratings at the 1st Bragg angle. The gas mixture was controlled with various argon and nitrogen gas pressures.
The fluorescence spectrum of Ar atoms shown in Fig. 2 was integrated by 20 pulses to reduce the error caused by pulse-to-pulse energy fluctuation. The lower black curve indicated the fluorescence spectrum from two TH filamentary plasma grating in pure Ar gas with the pressure of 0.5 atm. Only a few of neutral atomic lines were observed. With FW filament injected into the plasma gratings, these preexisted atomic lines increased dramatically. Furthermore, a host of additional ionic Ar lines appeared on a continuum background resulting from the Bremsstrahlung emission, as shown by the middle blue curve. This could be ascribed to heating of liberated electrons driven by the infrared filament inside the plasma channels. Electrons were liberated from Ar atoms through multiphoton absorption of ultraviolet photons due to its relatively low ionization (15.759 eV), which underwent electron acceleration by the subsequent FW laser electric field, and acquired kinetic energy scaled as I λ 2 . Accordingly, the electron density grew exponentially after the passage of the FW pulses as ne (td , t ) = ne (td , 0) exp(σ It / U i ) , where the initial seed-electron density ne (td , 0) and the inverse Bremsstrahlung cross section σ are both proportional to gas pressure P [18], I and t denote the intensity and duration of the heating FW pulse. Finally, numerous neutral argon
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Received 21 Jan 2014; revised 24 Mar 2014; accepted 23 Apr 2014; published 30 Apr 2014 5 May 2014 | Vol. 22, No. 9 | DOI:10.1364/OE.22.010968 | OPTICS EXPRESS 10970
atoms were subsequently kicked into the ionic states as a result of atomic collisions with these energetic electrons Ar+e → Ar + +e+e.
(1)
More high-lying excited states were populated through ion conversion accompanied by dissociative recombination [19] Ar+Ar + +Ar → Ar2 + +Ar; Ar2 + +e → Ar+Ar* ,
(2)
through which Ar atoms were populated in the excited state 4p (Ar*).The electron ion collision could also excite the Ar ion to the highly excited state through the process as Ar2 + +e → (Ar + )* +Ar+e.
(3)
This gives rising to Ar ionic fluorescence with the emission intensity proportional to the square of the gas pressure.
Fig. 2. Measured fluorescence spectra from TH plasma gratings in 0.5 atm Ar (lower black curve), from TH plasma gratings with Bragg incident FW pulses in 0.5 atm Ar (middle blue curve) and in 0.5 atm Ar mixed with 0.5 atm N2 (upper red curve).
When subsequently injecting N2 gas into the chamber, significant decrease of Ar atomic fluorescence was observed, as shown in Fig. 2 (the upper red curve). In order to reveal the mechanism of this observation, we studied the dependence of the continuum and argon atomic fluorescence intensity on the N2 pressure. As shown in Fig. 3(a) (red circle), the FI of Ar atomic characteristic line centered at 706.7 nm decreased rapidly when the first 0.1 atm N2 gas was injected in the chamber. All the characteristic lines from 600 to 1020 nm arisen from the 4p→4s transitions of the neutral Ar atom exhibited a similar decline with the FI decrease factor varied from 5 to 9. Further increase of N2 gas pressure above 0.1 atm, the FI only showed a slight decline. The distinct decline difference at different N2 gas pressure hinted competition between different collision mechanisms. As the inverse Bremsstrahlung cross section σ of electron-Ar is larger than that of electron-N2, ( σ = 2.94 × 10−19 cm 2 for Ar and σ = 2.05 ×10−19 cm 2 for N2) [20, 21], only a small amount of N2 injection (with about one quarter of Ar gas pressure in our experiment) was sufficient to saturate the rapid decline of the Ar atomic excitation by
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Received 21 Jan 2014; revised 24 Mar 2014; accepted 23 Apr 2014; published 30 Apr 2014 5 May 2014 | Vol. 22, No. 9 | DOI:10.1364/OE.22.010968 | OPTICS EXPRESS 10971
electron-argon impact. Further increase of the N2 gas pressure may influence Ar atomic impact excitation through collisions of Ar and N2.
Fig. 3. (a) Measured intensity of integrated continuum background (blue squares) and argon fluorescence spectra at 706.7 nm (red circles) from the mixture gases as a function of nitrogen gas pressure with argon gas fixed at 0.5 atm. Inset, the estimated electron density as a function of nitrogen gas pressure with argon gas fixed at 0.5 atm. (b) Measured intensity of ionic argon fluorescence spectra at 358.8 nm (red circles), atomic nitrogen spectra at 904.9 nm (blue squares) and ionic nitrogen spectra at 500.5 nm (olive triangles).
The FI decrease of atomic argon (Ar*) suggested that dissociative recombination played a dominant role with a small amount of nitrogen in the Ar-N2 gas mixture [22], Ar* +N 2 → Ar+N+N* ,
(4)
Ar + +N 2 → Ar+N 2 + ; N 2 + +e → N+N* .
(5)
The above reactions hinted that there should be two mechanisms in the dissociation of N2 inside the mixture of N2 and Ar gas. The first 0.1 atm N2 was apt to combine with the existed Ar* and Ar+ instead of directly dissociated by the laser beam. The decline of continuum background intensity as shown in Fig. 3(a) (blue squares) revealed the decrease of electron density [23], confirming the existence of the reaction (4) and (5). The estimated electron density was shown in the inset of Fig. 3(a). The initial electron density was about 1.06 × 1019 cm−3 when the cell was filled with 0.5 atm Ar, and after the injection of 0.1 atm N2, the electron density decreased to 0.9 × 1019 cm−3. However, when the pressure of N2 gas continued to increase, multi-photon absorption took advantage in this process which brought continuous increasing of the electron density. The increase of integrated continuum background intensity with more than 0.1 atm N2, exhibited in Fig. 3(a) (blue squares), was ascribed to the high-density electrons which were liberated from N2. Meanwhile, the recombination between N2 and Ar* or Ar+ always existed which could be clearly seen from Fig. 3(b) (red circles), the FI of Ar+, we take 358.8 nm (4d→4p) for example, kept to decrease with the increase of N2 pressure. The nitrogen atoms were also kicked into ionic states which promoted the avalanche ionization of neutral atoms and exponential increase of free-electron density: N+e → (N + )* +e+e.
(6)
With numerous N+ by energetic electron collision, a substantial amount of nitrogen atoms was populated into high-lying excited states through ion conversion and dissociative recombination processes N+N + +N → N 2 + +N; N 2 + +e → N+N* .
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(7)
Received 21 Jan 2014; revised 24 Mar 2014; accepted 23 Apr 2014; published 30 Apr 2014 5 May 2014 | Vol. 22, No. 9 | DOI:10.1364/OE.22.010968 | OPTICS EXPRESS 10972
The intensity of nitrogen atomic fluorescence (904.9 nm, 3p→3s, blue squares) and ionic fluorescence (500.5 nm, 3d→3p, olive triangles) increased with the nitrogen pressure. As shown in Fig. 3(b), the measured intensity of nitrogen atomic fluorescence exhibited a nitrogen gas pressure dependence scaling as P , suggesting the observed FI was proportional to the total number of nitrogen atoms. This further confirmed the dissociative recombination processes (6) and (7). We further investigated the dependence of fluorescence intensity on argon gas pressure while fixed the nitrogen pressure at 0.5 atm, as shown in Fig. 4. Although the FI of argon increased with its pressure (red square and blue triangle), as shown in Fig. 4(a), it was significantly weaker in comparison with the FI of pure argon at the same pressure, confirming the existence of reaction (4) and (5). Figure 4(b) shows that the nitrogen atomic fluorescence at 904.9 nm (blue square) and nitrogen ionic fluorescence at 500.5 nm (red circle) increased rapidly, which could be understood from electron-atom collision induced ionization [24] since the electron density increased rapidly [black circle in Fig. 4(a)]. The Ar atomic and ionic fluorescence intensity exhibited an Ar gas pressure dependence of P 2 , confirming the dissociative recombination processes (2) and (3). The linear dependence of nitrogen FI upon the Ar gas pressure as shown in Fig. 4(b) was consistent with the processes (4-6).
Fig. 4. Measured intensity of spectra emission from the gas mixture of 0.5 atm fixed nitrogen gas pressure and various argon gas pressure: (a) integrated continuum background (black circles), argon atomic fluorescence spectra at 706.7 nm (blue triangles), argon ionic fluorescence spectra at 358.8 nm (red squares); (b) nitrogen atomic fluorescence spectra at 904.9 nm (blue squares) and nitrogen ionic fluorescence spectra at 500.5 nm (red circles).
The characteristic fluorescence intensity of the gas mixture was also experimentally investigated under irradiation of FW laser pulses with different pulse energies, as shown in Fig. 5(a). Here, we chose ionic nitrogen fluorescence spectra at 500.5 nm as an example. The blue circles show the nitrogen fluorescence intensity versus the FW pulse energy, with its nonlinear fit according to F ∝ I × e I [24], where I denotes laser intensity. The black squares represent the measured intensity of the continuum background, which was proportional to the free electron density ( ne ), and could be well described by ne ∝ e I [25]. We next studied the FI dependence on time delay td . The fluorescence spectra from the argon and nitrogen gas mixture were measured at the same FW laser pulse energy (8 mJ) and gas pressure ratio (0.5 atm: 0.5 atm) as the time delay between the FW and TH pulses was tuned, as shown in Fig. 5(b). Here we also measured the nitrogen characteristic line at 500.5 nm for example. When the incident FW pulse arrived before TH beams ( td < 0 ), almost no fluorescence spectra were observed, indicating that the use of preformed plasma gratings to
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Received 21 Jan 2014; revised 24 Mar 2014; accepted 23 Apr 2014; published 30 Apr 2014 5 May 2014 | Vol. 22, No. 9 | DOI:10.1364/OE.22.010968 | OPTICS EXPRESS 10973
guide the infrared intense laser pulses was essential for the seeded electron acceleration and impact excitation of neutral atoms. The enhanced fluorescence was observed only when the FW and TH pulses were temporally overlapped, which was confirmed by the spike sharp of fluorescence intensity at td ~0 . This could be attributed to two-color fields induced ionization enhancement. The fluorescence intensity exhibited a slow monotonic decay for td longer than 2.3 ps. This should be resulted from the decrease of seed electron density induced by electron-ion recombination. The gradual decrease could be well described by [1 ne (2.3, t) + β (td − 2.3)]−1 , as fitted in Fig. 5(b), with time t and td scaling in picosecond unit, the plasma decay constant β ≈ 0.043 ps −1 , where ne (2.3, t) denotes the electron density after the passage of the FW pulse with a time delay of 2.3 ps. Such a delay was sufficient for free expansion of seed plasma [16].
Fig. 5. (a) FW pulse energy dependence of the background continuum intensity (black squares) and nitrogen fluorescence spectra at 500.5 nm (blue circles). (b) Experimentally measured dependence of nitrogen fluorescence intensity on the time delay between the FW and plasma-grating-forming TH laser pulses in the nitrogen and argon gas mixture.
3. Conclusion
In conclusion, we demonstrated dissociative recombination of argon and nitrogen gas mixture in filamentary plasma gratings. As argon atom and nitrogen molecule have comparable ionization potentials, electrons liberated from argon atoms and nitrogen molecule played almost the same role in the gas mixture. With intense infrared femtosecond pulses coupled into the preformed plasma gratings to heat the liberated electrons, electron-atom and atom-molecule collisions assisted dominant dissociative recombination that excited argon and nitrogen atoms into high-lying excited states. This may stimulate promising spectroscopic applications in the remote monitoring of atmospheric conditions. Acknowledgments
We acknowledge financial supports from the National Natural Science Fund (11274115 and 10990101), National Key Project for Basic Research (2011CB808105), National Key Scientific Instrument Project (2012YQ150092), and Innovation Program of Shanghai Municipal Education Commission (2014Z10269011).
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Received 21 Jan 2014; revised 24 Mar 2014; accepted 23 Apr 2014; published 30 Apr 2014 5 May 2014 | Vol. 22, No. 9 | DOI:10.1364/OE.22.010968 | OPTICS EXPRESS 10974
