thermal lens is followed by sampling the beam intensity with the pinhole-photomultiplier configuration. Intensity measurements at the instant the shutter opens, Abstract. Both a single beam and a dual beam (with synchronous detection) thermal IO, and when the steady-state condition is lens technique have been employed in the measurement of "colorless" organic com- achieved, Is, are used in calculatltig the abpounds in the range 1S,700 to 17,400 cm-'. Combination overtones of C-H stretching vi- sorptivity according to brations in benzene have been identified and agree with previous results obtained by con- o -is ai Is) JXkAk (2) ventional spectroscopy with a long optical path. Extinction coefficients as low as I x 10-6 Is Pl(dnldl') liter mole-' cm-' have been accurately determined. The sensitivity of the technique has been further demonstrated by measuring the So-T, absorption of anthracene; the spec- where x is the wavelength of the incident trum compares favorably with results obtained by conventional techniques. light (cm) and all other symbols are as before. We report the application of the thermal lens is divergent. For the steady-state limit, The characteristics of the thermal lens lens technique to absorption spectroscopy. where the heat deposited by the laser is depend not only on the laser mode but also The method is based on the temperature balanced by the outward conduction, the on the power absorbed in the liquid and the rise in an illuminated liquid induced by the focal length of the thermal lens in centime- exposure time, both of which should be absorption of small amounts of energy ters is minimized to avoid convection and disfrom a laser beam passing through the tortion effects on the lens (7). One must = JrJkw2 (1) sample. The localized temperature change also ensure that the laser maintains a Pal(dn/dT) brings about a transverse gradient in the TEMOO mode. index of refraction, which can be probed where J is Joule's constant (4.18 joule/cal), We have used the single beam technique optically as a "thermal lens." In recent k the thermal conductivity (cal cm-' sec-' to accurately measure absorptivities (8). years such thermal lensing has been well OK-'), w the beam waist (cm), P the inHowever, this is a d-c method, which is not characterized (1, 2), and it has been shown cident laser power (watts), a the absorptiv- only time-consuming but also inherently that under optimal conditions the optical ity (cm-'), and I the sample length (cm). less sensitive than methods which take adproperties of the lens can be simply related By measuring the strength of this steady- vantage of synchronous detection. Thereto the absorption coefficient of the mole- state thermal lens, Gordon et al. (1) calcu- fore, we have also designed and concule. lated the absorptivity of the liquid. structed a dual beam synchronous inHu and Whinnery (3), Whinnery (4), Refinements in the measurement tech- strument (9), which lends itself to continand others (5) have applied this method to nique were developed by Hu and Whinnery uous recording (10). absorptivity measurements of a number of (3). Our apparatus, derived from their Figure lb shows a schematic of the dual compounds at a fixed frequency-the work, is shown schematically in Fig. la. beam experiment. The chopped heating 632.8-nm line of the He-Ne laser. Their re- An 8-watt argon ion laser pumps a contin- beam forms a continuously pulsating thersults show systematic variations of the ab- uous wave rhodamine 6G dye laser. Ap- mal lens in the sample. The He-Ne.probe sorptivity within a particular homologous proximately 25 mw (- I percent of the to- beam passes through the sample and sensseries. However, measurements at a single tal output power) is delivered to the sample es the lens. The heating beam is blocked wavelength are not sufficient to properly cell, which is placed one "confocal length" from the detector by a cutoff and a narrow characterize the absorption. This problem (6) past the focus of a long focal length bandpass filter. The modulated probe seemed ideally suited for an argon ion lens. Hu and Whinnery demonstrated that beam is then synchronously detected. This pumped tunable dye laser system. We have this configuration allows the thermal lens signal is corrected for the heating beam used a tunable laser-based thermo-optical to exhibit the maximum defocusing effect power by a separate measurement. spectrometer to investigate weak absorp- on the laser beam. The laser beam expands Figure 2 shows the absorption of neat tions in organic compounds in the liquid onto a target containing a 500- ,m pinhole berizene over the tuning range of the dye phase. We report here some preliminary at its center. The time development of the laser determined by each of the two methresults that demonstrate the usefulness and ods discussed above. A 6.8-cm cell was extremely high sensitivity of the technique. used for the single beam experiment and a Sample -I The thermal lens effect was first report1.0-cm cell for the dual beam experiment. Laser oeV IL AMirror The ed and discussed in detail by Gordon et al. absorptivity at the He-Ne laser line Shutter Lens Heating (1), whose analysis forms the basis of the probe (15,800 cm-') agrees well with previously source present work. A liquid sample is placed in reported values (4). The dramatic failure of the beam of the laser. The liquid is heated perdeuterobenzene to reveal any lens efPinhole by the absorbed power and, in the absence in contrast to ordinary benzene, rules fect, To scope Detector of heat conduction. a transverse temper- a out electronic absorption as the cause of ature profile is established which matches this absorption. It does, however, strongly Bea m splitter the intensity profile of the laser. If the implicate higher order (overtone and comLaser li mode of this beam is TEMO., the intensity Probe sourc bination) vibrational transitions involving Lens Sample Pinhole profile is Gaussian. Sufficiently close to the hydrogen motion. In fact, thermal lensing Chopper center of the beam, the laser intensity prostudies of various substituted benzenes detection file is parabolic and, even after heat conshow a dependence of the intensity of this duction takes place, so is the temperature peak on the number of ring hydrogens (11). Laser profile. Since most liquids have a positive The peak location agrees very well with the Prism _ coefficient of thermal expansion, the tem- b fifth overtone of the C-H stretching vibraHeating source perature coefficient of the index of refrac- Fig. 1. Experimental setup: (a) single beam ap- tion reported in 1928 by Ellis (12), who tion, dn/dT, is negative, and the thermal paratus and (b) dual beam apparatus. used conventional spectroscopy with cells

Thermal Lens Technique: A New Method of Absorption Spectroscopy

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up to 7 m long. The thermal lens method sorptivities. Stone (14) described an interprovides an accurate determination not ferometric technique that senises the only of peak location but also of absorp- change in optical path length accomtion strength without complications found panying a temperature rise in the solution in conventional low absorbance measure- induced by absorption from a high-inments. tensity arc lamp. The low intensiities and A reasonable extension of this thermal therefore poor spectral resolutiorn achievthis lens technique is the determination of sin- able with an incoherent source I glet-triplet absorption spectra. Figure 3 method. Other workers (15) have explored .vised by shows the singlet-triplet absorption spec- the optoacoustic technique, first d4 evised trum of a 10-2M solution of anthracene in Bell (16) in 1880, which senses pressure ethyl iodide measured in a 10-cm cell. The changes induced by absorption from a arrows refer to a shoulder, valley, shoul- modulated light source. This met}iod isd 1S didlder, and peak (left to right) seen for the rectly applicable to measurements of weak same system by conventional spectroscopy absorption in gases and can be useA on solusing a 20-cm cell (13). A higher order C- ids and liquids enclosed in a prcessurized H vibrational transition of anthracene is optoacoustic cell. Pinnow and Rtich (17) expected at 16,500 cm-' in the region of the have described a calorimetric me thod for first singlet-triplet transition, and it is pos- measuring optical absorption in bulk solsible that this causes some of the activity ids. By placing their calorimeter inside a observed both in this and the earlier work (13). Isotopic substitution of anthracene would distinguish between the two types of transitions. We can extrapolate the ultimate sensi- (n 3 tivity of the thermal lens technique from our measurements on the overtone absorp- >1 tion spectrum of benzene. Using only 1 T percent of the available laser power and 1- , 2 cm cells, we measured molar extinction as c low as 2 x 10-6 liter mole-' cm-'. With full a laser power, 5-cm cells, and better-stabi- ,