Coherent anti-stokes raman scattering in gaseous nitrogen with picosecond pulses

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COHERENT ANTI-STORES RAMAN SCATTERING IN GASEOUS NITROGEN WITH PICOSECOND PULSES* B-1. GREENE, R_B_ WEISMW

and R.M. HOCHSTRASSER

Deportment of Chemishy and Laboratory for Resear& on the Structure of Matter. University of Pennsylvani& Phifadephia. Pennsylvania 19104. USA Received 20 July 1978

The use of CARS as a probe of gas phase molecular concentrations has been extended into the picosecond regime. A double-beamconfiguration employing singe pulses from a mode-locked Nd:glass laser has allowed measurement of Nz (‘Q, u = 0) pressures to an accuracy of better than 10% with 8 ps time resolution. This technique shows promise for the study of dy%unics of gas phase photodissociation processes

1. liltroduction Whenever two waves at frequencies wl and w2 are incident on an isotropic medium an oscillating polarization is generated by the nonlinear response of the medium to the fields. The lowest order nonlinearity involves the third order susceptibility xt3)- One frequency ccmponent of the polarization occurs at w3 = 2q - 02 through xt3) (--03, w1, wl, 44 and the process is termed three-wave mixing [l] . This three-wave mixing process can be resonance enhanced in many ways [2], but if in particular tt(ol - ~2) is chosen to be positive and equal to the separation between two vibrational energy states of the medium then any enhancement of the signal at o3 is usually termed a CARS process_ Studies of CARS in gases were carried out by Regnier and Taran [3] _ The object of this and subsequent studies [4-61 was to explore the analytical capabilities of CARS, especially in relation to detecting trace amounts of one species in the presence of another_ The principal limitation on detectability was shown to be the signal at 03 generated by the nonresonant real part of the susceptibility contributed by the carrier gas_ Since *be QThisressh was supported by a grant from the Nationai Science Foundation (CHE7C84428) and in part by the MRL Program under Grant No. DMR76-80994. z;hNSF National Needs Past-Doetoti Fellow.

w3 beam is generated fro,m the coherent polarization source it is itself coherent and easily spatially isolated from incoherent radiation emitted from the medium being studied_ Thus CARS has proved very useful in the study of flames [4,7,8] and the vibrational energy distributions of diatomics in flames_ The present study was initiated to explore the possibility of using picosecond pulses from a glass laser to measure by a CARS process the concentration of nitrogen molecules in the gas phase, It is not generally simple to detect and state-analyze small inert molecules generated in photcchemical reactions_ Such measurements must be made in collision free systems and for homonuclear diatomics would appear to require molecular beam met&Is_ Since the CARS observation can be made within a few picoseconds after optical excitation, collision free measurements may be made in moderately high pressure systems (e.g., ca. 104 torr for excitations requiring a hard sphere co&ion to bring about energy transfer)_ Techniques similar to those described below may also be employed to study time-resolved CARS in condensed phases_

2. Experimental

metnua and results

The source of picosecond light pulses used in this work was a passively mode-locked Nd : glass laser system. Briefly, this consists of an oscillator cavity

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with an Owens-lllinois ED-2 rod, a contacted flowing dye cell, an-aperture for control of transverse mode structure, and a type I phase matched KDP crystal for intensity stabilization by means of intracavity second harmonic generation [9] . A single pulse was selected from the output pulse train with a Pockels cell extraction system and then amplified in two double-passed amplifier rods. The resulting 1.06 m pulse had an energy of approximately 30 mJ, a duration of 8 ps, a second harmonic spectral width of 15 cm- l, and a gaussian transverse intensity distribution_ Second harmonic generation with a second KDP crystal gave a light pulse containing several millijoules at 530 mn. This 530 mn beam served as our o1 frequency in the CABS process_ Lacking a tunable picosecond light source, we chose to use stimulated Raman scattering in liquid nitrogen as the method for generating the 02 beam. Although the vibrational transition frequency in liquid nitrogen (2326 cm- l) differs somewhat from the gaseous Q-branch value (233 1 cm-l), the two are separated by less than our her bandwidth. Fig. 1 schematically shows the experimental configuration after production of the 530 run picosecond pulse. A lens of $3 cm focallength focuses the 5 mJ second harmonic light into a dewar of liquid nitrogen having a 6 cm pathlength. After collimation, the anti-Stokes and bigher order Stokes

UQ N,

Fig. 1. Schematic diagam of the double--beam CARS effect apparatus_ Abbreviated elements are: F, filters; L.,lenses; BS, beam s@ter; M, monochromatoa; PMT_ photomultipliers.

1 November 1978

components were removed with Schott OG-5 15 and dielectric 640 mn short-pass filters. Conversion efficiency to the fmt Stokes wavelength (605 MI) was typically 2%. The w1 and w2 beams, which overlapped spatially and temporally, passed through a 20% reflecting beamsplitter and were focused with a 7 cm focal length lens into a 10 cm pressure cell connected to the regulator of a gaseous nitrogen cylinder_ Anti-Stokes light at 472 mn which was generated via the nonresonant third order susceptibility of the pressure cell’s entrance window was blocked by an GG-5 15 filter placed just inside the window., Similarly, a BG-25 fnter in the cell, near the exit, transmitted the CARS light while preventing the laser and Stokes beams from reaching the second window. For enhanced spectral discrimination against the intense w1 and o2 beams, the light from the pressure cell passed through a 520 run short-pass dielectric filter into a %-meter single monochromator used with a 300 groove/mm grating in first order, set at the 472 run CABS wavelength. The signal pulse was detected with an EM1 6256s photomultiplier whose photocurrent was integrated in the capacitance of a I-meter coaxial cable terminated in 20 kfi at the vertical input of a dual beam oscilloscope_ The sweep of this instrument was triggered by the gate output of a second, wide bandwidth storage oscilloscope used to monitor the laser p-ulsetrainfor each shot. In order to compensate for shot-to-shot fluctuations in the laser output, a double-beam optical arrangement was employed_ Partial reflections of the 01 and 02 beams taken before the sample cell were focused with a 4 cm focal lengrh lens into a second 6 cm pathlength dewar of liquid nitrogen_ We note that this focusing cone was large enough to allow some phase matched CABS generation even for our collinear geometry_ The emergent light was filtered with Corning 5-57 and Schott BG-25 filters and focused into a Beckman prism monochromator fitted with a IP28 photomultiplier. The output of this detector was displayed in the same way on the second beam of the dual beam oscilloscope. In this way two traces could be observed for each laser shot: one representing the integrated CABS signal from the sample in the variable Pressure cell, and the o*&er showing the reference CARS magnitude from the fmed-density liquid nitrogen_

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Several tests provided verification that the observed signals indeed resulted from a CARS process in gaseous Nz_ First, blocking eitber tbe Stokes or the laser frequency with added filters caused the signal to van&. in addition, tuning the monocbromator away from tha expected anti-Stokes wavelength resulted in a signal decrease, thus confirming the x(3’ origin of the light. Finally, flusbing the sample cell with Oz or Ar gave a large reduction intire anti-Stokes signal. We took this residual value to represent the level of non-resonant background for our cell and optical system. When the normalized and background corrected anti-Stokes magnitudes were measured as a function of nitrogen pressure, the data shown in fig. 2 were obtained. This log-log plot, constructed for PO = 1030 torr, shows a linear least squares slope of 1.97 in excellent agreement with the theoretical value of 2.00 for a CARS process in the low density limit. At pressures of less tban 1000 torr we find that the non-resonant background contribution from the

t

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apparatus dominates our total signal, thereby decreasing the accuracy of tbe CARS measurement. Using a Princeton Applied Research Corp. model 220%/D optical multichaunel analyzer in conjunction with a %-meter spectrograph, we measured the spectral distributions of the laser second harmonic and Stokes-shifted beams after the first liquid nitrwDen cell. When the incident 530 nm pulse was attenuated to power levels of lo7 W or less, its spectral width remamed essentially unchanged and there was no signiticant stimulated Raman generation_ However, at power levels above lo8 W, for which efficient stimulated Raman shifting occurred, we observed extreme spectral broadening of the laser pulse to widths in excess of 100 cm-l. Fig. 3 illustrates this effect with representative spectra from the low and high power regimes. The Stokes spectra were observed to be similarly broadened.

12-

d

/ 0

IO-

0 4 GIGANATT

d

08,o

/

3j 506-

/p

SiOPE

= I 97

0

I 5 MEGAWATT

536

0

01

02

03

04 LOG (P/p,

05

06

07

1

Fig_ 2, Log-log plot of Ns gasphaseCARS signalversus

samplepressure.po is 1030 toIT_

534

532

WAVELENGTH

530

528

, nm

Fig. 3. Singlepulse spectraof the laser second harmonic

beam following the liquid ni’togen Rarnanshiftingceil. Upper trace is for an incidentpulse power of 0.4 GW; lower trace is for 1.5 MW.

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3. Discussion

We have succeeded in making ObseNatiGns of CARS signals from gaseous nitrogen molecules in v = iI states_ These measurements have been accomplished with light pulses of ca. 8 ps duration and thereby establish a method of probing for NT within a few picoseconds following pulsed excita%on of the system. A main factor limiting the strength Gf the o3 signal in this experiment is the spectral linewidth of the incident fields_ The resonant part of x(3) in our system has the form:

where C isa constant, p is the gas pressure (all molecules are assumed to be in the v = 0 state); PJ is the fmcEior& population of the rotational state p; aJ is the frequency of the transition I vJ > = I CJ) + I 1J) corresponding to the Q-branch transitions that Probably dominate our signal; and D, is a damping factor for the specified transition. In effect WI and w2 correspond to particular Fourier components of the incident fields, and the total polarization Pat 633 is generated by adding contributions from all the Fourier componerits, each being weighted by the form of(l), through [IO]

P(@

=sdw;

dw’; dw; x(~‘(-w~,

0; , w’i,

-w;>

Obviously there is a frequency distribution in 03 _ P (~3) is a source for the Fourier component E(9); the detector bandwidrh will determine which of these components contribute to the observed signal intensity iE(9) l* _ The detected intensity at any one frequency is proportional to the convolution of the intensity versus frequency protIles of the WI and W-Jbeams whkh adds up all contributions for which o’ 1 - W’2 = Ou. Assuming TJ t0 be Very small compared with all other widths, we may write: r(o3? = C P*~I(GJ~ - mr>sdwi;

(0 + a,>r,(G~), (3?

where 11 and 12 are normalized intensity profiles peaked at w1 and w2 respectively. 8

1 November 1978

picosecond pukes have a spectral width that is large compared with the Doppler or collision broadening in gases at moderate pressures. In our experiments the laser pulse was not transform limited (corresponding to ca_ 3 cm-l) but was instead about 15 cm- 1 wide. Thus the mismatch of a few wavenumbers between the Q-branch frequency and the difference in the peak frequencies of the w1 and w2 beams should have resulted in a relatively small !oss of a factor of 2 or 3 in signal stren,@h at OS_ However, as described in section 2, at the higher powers the laser and Stokes beams become severely spectrally broadened. It is this effect which principally limits our CARS signal magnitudes. Therefore when the present technique is employed there is little to be gained by increasing the laser power beyond a few megawatts, as the amplitudes of the useful Fourier components of the incident fields are reduced by selfphase modulation and other nonlinear effects [ 111. Within the range of sample pressures shown in fig. 2, normalized signal magnitudes allow determination of the nitrogen ground state number density to an accuracy of 5 to lC%. This level of reliability results from the double beam configuration, which effectively compensates for shot-to-shot fluctuations in the laser pulses. We note that the quadratic dependence of signal strength on gas pressure observed for our experimental conditions is expected only for relatively tight beam focusing; otherwise the signal should reflect the variation of coherence length with pressure as recently shown by Miles et al_ [12]_ This method for the accurate time-resolved measurement of molecular concentrations in the gas phase should enabie significant new studies of isolatedmolecule photodissociation dynamics_ For example, a picosecond photolysis pulse can be arranged to excite the sample at various times before the CARS pulses probe for the photofragment density. Because of the short time scale involved, high sample pressures may be used without introducing collisional effects. The present detection sensitivity should be adequate for the srudy of samples with favorable quantum yields and cross sections_ Addirional sensitivity and selectivity will be obtained by the replacement of Raman shifting with a tunable source, such as a synchrqnously pumped dye laser [ I31 , for the generation of the w2 beam. In this way the

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spectral broadeningeffect can be avoided and (wl - w2) can be tuned exactly to a resonance of interest, including excited as well as ground state transitions.

LETTERS

References N. Bloembergen, Nonlinear optics (Benjamin, New York, 1965).

[21 C_ Fiytzanis, in: Quantum electronics, Vol. 1. Noulines.r optics A, eds. H. Rabin and CL. Tang (Academic

[31 [41

Press, New York, 1975) p_ 9. P. Regnier and J.P.E. Taran, in: Laser Raman gas diagnostics, eds. 81. Lapp and CM. Penney (Plenum Press, New York, 1974) p. 87. F. iMoya, S.kJ. Druet and J.P.E. Taran, Opt. Commun_ 13 (1975) 169.

1978

[Sl W.B. Roh, P. Schrerber and J.P.E. Taran, AppL Phys. Letters 29 (1976) 174. 1’51 F. hioya. S.A_J_ Druet and J-P-E. Tar-an, in: Laser

[71

Ill

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[91 El61 IL11

r121 1131

spectroscopy, eds. S. Haroche, J.C. Pebay-Peyroula, T.W. Hansch and SE. Harris (Springer, BerBn, 1975) p_ 66. J-IV_ Nibler, J-R. McDonald and A.B. Harvey, Opt. Commun 18 (1976) 37l_ W.M. Shaub, J.W. Nibler and A.B. Harvey, J. Chem. L’hys. 67 (1977) 1883. R-B. Weisman and S.A. Rice, Spectry. Letters 8 (1975) 329_ R.M. Hochstrasser, G.R_ hieredith and H.P. Trommsdorff, Chem. l’hys. Letters 53 (1978) 423. D.H. Auston, in: Topics in applied physics, Vol. 18. Ultrashort light pulses, ed. S-L. Shapiro (Springer. Berlin, 1977) p. 171. R-B. Miles, G. Laufer and G.C. Bjorkhmd, AppL Phys. Letters 30 (1977) 417. T-R. Royt, IV-L. Faust, L.S. Goldberg and C.H. Lee, AppL Phys. Letters 25 (1974) 514.

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