April 1979 / Vol. 4, No. 4 / OPTICS LETTERS 103

Time-resolved infrared spectral photography

D. S. Bethune, J. R. Lankard, and P. P. Sorokin

IBM Thomas J. Watson Research Center, P.O. Box 218, Yorktown Heights, New York 10598

Received December 13, 1978

A new technique for time-resolved (5-nsec) recording of broadband infrared absorption spectra is demonstrated.Resonantly enhanced, third-order nonlinearities of metal vapors are used both to produce a pulsed infrared contin-uum that interrogates the sample and to upconvert this infrared continuum into the visible. Single-shot absorp-tion spectra of CO2 and H20 bands in the region 2.55-2.85,um were obtained.

Techniques for high-speed recording of broadbandinfrared spectra of molecules in the so-called "finger-print" region (2-20 Aim) are limited, despite the obviouspotential value of such methods in characterizingchemical transients produced in flash photolysis,laser-induced chemical reactions, etc. Photographicemulsions are basically insensitive at wavelengths longerthan about 1 gm. At shorter wavelengths, only highovertones of fundamental vibrational modes of mole-cules, which are orders of magnitude weaker in strengththan the infrared fundamentals, are observable in ab-sorption. In this Letter, we demonstrate a new tech-nique for photographic or multichannel photoelectricrecording of broadband infrared absorption spectrahaving the following essential characteristics: timeresolution -5 nsec, spectral resolution -0.5 cm-1 ,spectral range interrogated per shot -400 cm-1, andcentral frequency potentially variable in several rangesin the 2-20 gm region. An infrared spectrum can bephotographed in a single shot, even without the use ofimage intensifiers.

The method utilizes resonantly enhanced third-ordernonlinearities of metal vapors,' both to produce a pulsedinfrared continuum that interrogates the sample andto unconvert this infrared continuum into the visible.The experimental setup is shown in Fig. 1. We observethat, when a broadband (-800-cm-l-wide), spectrallysmooth, horizontally polarized, -5-nsec-long, super-fluorescent visible-continuum beam (v,) of sufficientintensity is focused into a K vapor cell, a horizontallypolarized, -5-nsec-long, -400-cm- 1 spectrally wideinfrared continuum beam (Vir) is generated by the pro-cess of stimulated electronic Raman scattering (SERS).In this experiment the spectral breadths of both pumpand Stokes beams are more than two orders of magni-tude greater than in all previous studies of SERS. Thevisible continuum beam is generated by pumping a dyecell with all feedback removed and amplifying the col-limated fluorescence in a second stage to -200 kW.Figure 2(a) shows the spectrum of the beam v, trans-mitted through the first K cell when diphenyl stilbene(DPS) dissolved in p-dioxane and pumped by the thirdharmonic of a Quanta-Ray Nd3+:YAG laser is used togenerate the continuum beam. With a continuumpower of -200 kW, an SERS power of -2 kW was

measured, corresponding to -7% efficiency for con-version of quanta from vP to Vir.

The pulsed infrared continuum beam passes througha Ge filter and interrogates the sample. The infraredbeam transmitted through the sample (vir*) is then fo-cused into a second K vapor cell, where it is made tooverlap a vertically polarized, narrow-line, DPS dye-laser beam (vL), which is introduced via a Glan prism.The latter beam generates SERS in the second K cell,

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Fig. 2. (v increases to right.) {a) DPS continuum spectrumthrough -5-Torr K, photographed in first order; absorptionlines are K 4s-5p lines at 404.7 and 404.4 nm. (b) Upcon-verted DPS-K-K spectrum, photographed in first order,sample cell evacuated. (c) Same as (b), but with -500-TorrCO2 in sample cell.

0146-9592/79/040103-03$0.50/0 C 1979, Optical Society of America

104 OPTICS LETTERS / Vol. 4, No. 4 / April 1979

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Rb t +. t l K tlI . 1 ,T T

RbK 13 11 8

Fig. 3. (v increases to right.) (a) Upconverted DPS-K-K spectrum, photographed in second order, with CO2 in sample cell.Indicated band heads are 107 cm-' apart. (b) Upconverted POPOP-Rb-K spectrum, photographed in second order, samplecell removed. Dotted arrows show nulls, and numbers correspond to calibration lines in Fig. 4. Two exposures are shown.

producing a narrow-line, vertically polarized Stokesbeam (vs). In this second cell, vL, vs, and iir* beat tocreate, by four-wave mixing (Fig. 1, lower-left inset), ahorizontally polarized visible beam vp, which is trans-mitted through a second Glan prism and then recordedon a spectrograph plate. Since the upconverted in-tensity at vp = Vir* + PL - S = Pir* + AV4s-5 s is pro-portional to the infrared intensity at vir*, the devicetranslates the infrared spectrum of the sample to thevisible, where it can be photographed. Power levels upto 3 kW were measured for the upconverted beam at Pp,corresponding to a noteworthy photon upconversionefficiency from the infrared of -23%.

The dye DPS in p-dioxane was selected because itscontinuum overlapped the K 4s - 5p resonance lines.Since the Stokes beams produced in the alkalis by SERSnormally correspond to np - ns transitions, the np -ns frequencies determine the midpoints of the rangesin which infrared continua may be produced. For K,the wavelength of the 5p - 5s transitions is -2.7 ,m,and with DPS dye, an infrared Stokes continuum beamextending from 2.55 to 2.85 ,m is produced.

Figure 2(b) shows the general appearance of the up-converted spectra recorded on 1-N photographic plateswith a 1.5-m grating spectrograph used in first order,5-Torr K in both cells, and the sample cell nominallyevacuated. A relatively smooth spectrum -400 cm-'wide is observed. Two strong dark lines correspond toabsorption at the K 4s - 5P1/2,3/2 frequencies. In be-tween, roughly twice as close to the 4s - 5 P1/2 line asto the 4s - 5P3/2 line, there occurs a sharp dark line,which we attribute to a cancellation of the 5p1/2 and5P3/2 contributions to the Raman susceptibility. In thepresent case, the upconverted output is rigorouslyproportional to I X(3) xxyy 12, and for s - p - s Ramantransitions, X 3)xxyy = X 3)xxxx, the symmetric part of theRaman tensor. The observed cancellation in thesymmetric term of the Raman susceptibility betweenmembers of a resonance line doublet is a predictedfeature for all alkali metal atoms.", 2

The spectrum of Fig. 2(c) was taken with -500-TorrCO2. pressure in the 18-cm-long sample cell. Twowell-known C02-band systems are observed, the 0201band at 3609 cm-' and the stronger 1001 system. Theirrespective band heads, at 3609 and 3716 cm-', are in-dicated. In both cases the P and R branches are clearly

evident. The overexposed line obscuring part of the Rbranch of the 3609-cm-' system is incompletely nulledlight from the arbitrarily tuned laser at PL.

With the spectrograph used in second order to obtainhigher resolution [Fig. 3(a)], the rotational lines of theP and R Jbranches of the CO2 combination bands areclearly resolved. At the peak of the P branch of the3609-cm-' band, adjacent CO2 lines are known to bespaced -1.7 cm-' apart. The observed infrared reso-lution is -0.5 cm-', consistent with the resolution of ourspectrograph and the -0.5-cm-' linewidth of the tun-able laser. Note that VL has here been tuned completelyoff the 3609-cm-1 band. Tuning of tL over a wide rangedoes not greatly influence the appearance of the up-converted spectra.

For the upconverted spectrum of Fig. 3(b), a 5-TorrRb cell and phenyl-oxazolyl-phenyl-oxazolyl-phenyl(POPOP) in p-dioxane were used to generate the in-frared continuum, and the sample cell was completelyremoved. The upconverter and the narrow-line DPSlaser at VL were unchanged. The wavelengths of the Rb6p3/2,1/2 - 6s transitions are quite close to those of theK 5P3/2,1/2 - 5s transitions (see Table 1), so that similar

Table 1. Alkali Resonance Lines and CorrespondingInfrared Frequencies

Alkali Transition X(np) v,,X(np - ns)

Cs 7P3/2 - 7s 4557 A 3411 cm-' (2.93,pm)7P1/2 - 7s 4594 A 3230 cm-' (3.10 Mm)8P3/2 - 8s 3877 A 1475 cm-' (6.78 Am)8p1/2 - 8s 3890 A 1392 cm-' (7.18 ,m)

9P3/2 - 9s 3612 A 771 cm-' (12.96 Am)9P1/2 - 9S 3618 A 727 cm-' (13.76 pm)

10P3/2 - lOs 3478 A 454 cm' (22.04 ,m)

11P3/2 - 11s 3399 A 291 cm-' (34.35 Am)

Rb 6P3/2 -' 6s 4203 A 3659 cm-' (2.73 um)6p1/2 - 6s 4217 A 3582 cm-' (2.79 pm)

7P3/2 - 7s 3588 A 1559 cm'1 (6.42 ,m)

8P3/2 - 8s 3350 A 807 cm-' (12.39 ,m)

K 5P3/2 - 5s 4045 A 3693 cm-1 (2.71 pm)5P1/2 - 5s 4048 A 3674 cm-' (2.72 pm)

April 1979 / Vol. 4, No. 4 / OPTICS LETTERS 105

3900 3880 3860 3840 3820 3800WAVENUMBER (cm-l)

Fig. 4. Comparison of densitometer trace of portion ofspectrum shown in Fig. 3(b) (traced from original) (upper)with IUPAC calibration spectrum (lower).

regions in the infrared are probed in Figs. 3(a) and 3(b).In Fig. 3(b), dark bands corresponding to 4s - 5P3/2,1/2resonance line absorptions of K atoms in the second cell,and to absence of infrared light at the exact 6p3/2,1/2 >6s frequencies of Rb, are seen. One notes again thesharp interference null in I X(3)xxyy 12 for K produced bythe upconverter cell. A corresponding, more-diffusenull is seen between the Rb resonance lines.

The myriad of sharp, dark lines on the high-frequencyside of the spectrum in Fig. 3(b) results from residualwater vapor in the optical path between the two vaporcells. A densitometer trace of a portion of the spectrumin Fig. 3(b) is shown in Fig. 4. The correspondence witha published IUPAC water-vapor spectrum,3 Fig. 4(inset), is apparent.

For chemical diagnostic applications, it is importantthat other ranges of the infrared can be probed in thesame manner as demonstrated here for the 2.7-,um re-gion. Table 1 lists the frequencies and wavelengths ofvarious np - ns transitions of the alkali metals, to-

gether with the corresponding wavelengths of the npresonance lines. It is expected that sufficiently intensecontinuum beams, spectrally overlapping the np reso-nance lines indicated, will produce broadband infraredcontinua by SERS that span the corresponding rp -ns transition frequencies. How far each infrared rangecan be extended by shifting the dye continuum awayfrom the np resonance lines is uncertain because of theusual [VP - VP(np)]-2 variation in resonance Raman crosssection. However, with 1-10-mJ narrow-band excita-tion of Cs, tunable infrared has been generated by SERSover three ranges, 2.5-4.75, 5.67-8.65, and 11.7-15 gAm,

corresponding to the np - ns transitions for n = 7,8,9.2It has also been demonstrated experimentally4 andtheoretically5' 6 that, in a dispersionless medium, theRaman conversion efficiency and threshold pumpingintensity are practically independent of the total spec-tral linewidth of the exciting radiation. We thereforeexpect that, as the pump continua beams are detunedfrom np resonance doublets, this low dispersion ap-proximation may be a good one, and SERS will generatebroadband infrared with thresholds and overall tuningranges similar to those observed in the narrow-bandcase.

Work is in progress to extend the infrared probingrange and to apply this technique to a dynamicalchemical situation.

Note added in proof: We have recently photo-graphed the infrared region 2.8-3.65 gim using a di-methyl POPOP continuum, Raman shifted in Rb vapor.A K upconverter was again used.

This work was partially supported by the U. S. ArmyResearch Office.

References

1. See, for example, J. J. Wynne and P. P. Sorokin, "Opticalmixing in atomic vapors," in Nonlinear Infrared Genera-tion, Y. R. Shen, ed. (Springer-Verlag, Berlin, 1977).

2. D. Cotter, D. C. Hanna, and R. Wyatt, Opt. Commun. 16,256 (1976).

3. IUPAC Commission on Molecular Structure and Spec-troscopy, Tables of Wavenumbers for the Calibration ofInfrared Spectrometers (Pergamon, London, 1977), p.98.

4. V. V. Bocharov, A. Z. Grasyuk, I. G. Zubarev, and V. F.Mulikov, Sov. Phys. JETP 29, 235 (1969).

5. Yu. E. D'yakov, JETP Lett. 11, 243 (1970).6. See G. P. Dzhotyan, Yu. E. D'yakov, I. G. Zubarev, A. B.

Mironov, and S. I. Mikhailov, Sov. J. Quantum Electron.7, 783 (1977), and references therein.

WATER VAPOR

311111 : 8 11 I