881
J. Phys. Chem. 1993,97, 881-888
Quantum-State-Resolved Studies of Rovibrational Excitation of NzO and OCS following Collisions with Low-Energy Electrons Lei Zbu and George W . fly^' Department of Chemistry and Columbia Radiation Laboratory. Columbia University, New York,New York 10027 Received: July 13, 1992; In Final Form: November 2, 1992
The vibrational excitation of N20 and OCS following excimer laser photolysis of 12 in a low-pressure mixture of I2 and N2O or I2 and OCS has been studied using an excimer laser photolysis/diode laser probe technique. Vibrational excitation probabilities and nascent rotational distributions have been obtained for a number of low-lying vibrational levels. In addition, measurements of the transient N20 and OCS line widths have been performed for the majority of the rovibrational states probed. Although a relatively large amount of vibrational excitation has been observed in N2O and OCS,negligible energy transfer to the translational and rotational degrees of freedom of the products is observed. The results are consistent with the production of vibrationally excited N2O and OCS molecules via collisions with low-energy electrons arising from multiphoton ionization of 12.
I. Introduction
9ow
Electron-molecule scattering informatioin is important in studyingchemical reactions in the upper atmosphere, in optimizing gas laser performance, and in maximizing the efficiency of magneto hydrodynamicpower generation. There have been many studies of direct electron-molecule inelastic scattering and of the formation of temporary negative ion states.l-3 The typical electron-scattering experiment consists of energy-selected electrons from an electron gun colliding with a molecular beam, followed by detection of the scattered electrons with a rotatable energy analyzer. However, the conventional experimental technique is not capable of resolving the vibrational and rotational structures of most heavy molecules. As a result, the study of rovibrational excitation of molecules (especially polyatomic molecules) by electrons is still in its early stages. Quantumstate-specific investigations of rovibrational energy-transfer processes initiated by electron collisions in molecular gases are particularly important since they can reveal the details of the interaction mechanism (direct or resonant excitation) and provide comparisons with theoretical calculations. The use of narrow bandwidth lasers to produce monoenergetic electrons and to probe the excited molecular species following electron-molecule interactions holds much promise for providing new insightsinto the dynamicsof electron collisions with diatomic and small polyatomic molecules. Time-resolved high-resolution infrared diode laser absorption spectroscopy4-I0 allows the vibrational, rotational, and translational energies of molecules, excited by direct electron scattering or produced by decay of temporary negative ion states, to be determined. The e- + C02 scattering process has been studied recently using this technique!-10 In this paper, rovibrational excitation of N20 and OCS by electrons will be described. An energy level diagram for N 2 0and OCS vibrational states is shown in Figure 1. Studies of electron scattering by N2O and OCS are interesting since. these molecules have several unique properties for state-resolveddynamic studieswhen compared with C02. The molecules C02, N20, and OCS represent a series of closely related triatomic linear molecules. The similarity of their electronic ground-state configurations combined with the systematic variation of the electron affinity and dipole moments led us toexpect similaritiesand differencesin thevibrational excitation probabilities. In addition, strong Fermi resonance in C02 couples the loo0 (symmetricstretch) and0200 (bend overtone) vibrational states, complicating the interpretation of quantum-state distri0022-3654/58/2097-088 1$04.00/0
oa
= -
f
S
"i
low
0.1
OP
I
Figure 1. Energy level diagram showing the low-lying vibrational states of the N20 and OCS molecules. For clarity, combination levels are not shown.
butions involving these states. In N20 and OCS, the Fermi resonance between the low-frequency stretch (lo00 for N2O and W1 for OCS) and the bend overtone (0200) is much weaker than in C02. (In this section, we use the nomenclature of Hunt et a1.l I to label the vibrational levels of OCS where the high-frequency 2062-cm-l stretch is designated as V I , or 1o00, the low-frequency 859-cm-I stretch is v3,or W 1 ; and the 520-cm-I bending vibration is v2, or 01 IO.) Using the diode laser probe technique, the relative vibrational excitation probability can be determined separately for all three fundamental vibrations of NzO and OCS. In the present experiments, hot electrons, e-*, are prepared by excimer laser multiphoton ionization (MPI) of I2 at 193 nm:
-
+
1, nhv (193 nm) 1; + e-* (electron production) Rovibrationally excited N20 and OCS moleculesare produced by collisional excitation: e-* e-*
+ N,O (00'0;
+ ocs (OOOO;
J', V')
-+ e-
N 2 0 (mn'p; J , V)
- + ocs
J : V I ) e-
(mn'p; J , V) (collisional excitation)
where m, n, and p are, respectively, the number of quanta of low-frequencystretching, bending, and high-frequency stretching motions for N20 and the number of high-frequency stretching, CQ 1993 American Chemical Society
882 The Journal of Physical Chemistry, Vol. 97, No. 4, 1993
Zhu and Flynn
bending, and low-frequency motions for OCS; J and I are the rotationalend vibrationalangular momentum quantum numbers; and Vis the recoil velocity of the N20 and OCS molecules. Timeresolved diode laser absorptionspectroscopy is then used to monitor the nascent excited rovibrational states populated in the scattering processes:
within these levels. Ultrahigh-resolution studies of electronOCS and electron-N20 scattering using the infrared diode laser detection method, which can resolve all of thee quantum states as well as the molecular velocity recoil profiles, are described below.
N 2 0(mn'p; J, V) + hv (-4.5 pm)
The excimer laser photolysis/diode laser probe doubleresonance apparatus has been described in detail elsewhere and will be outlined here only briefly.e7 An excimer laser operating at 193 nm provides a photolysis pulse which is directed through a 2.76-m-long sample cell containing a 1/10 mixture of I2/0CS or 12/N2O at a total pressure of 27.5 mTorr. The cw infrared radiation from a lead-salt tunable diode laser at -4.7 pm (to probe0CS)or -4.5pm (toprobeN20)and thepulsedphotolysis beam are propagated collinearly through the sample cell via a dichroic MgF2 beam splitter coated for high reflectivity at 193 nm. A second dichroic beam splitter removes the UV light at the end of the cell, and the IR beam passes through a monochromator and then onto a liquid nitrogen cooled InSb detector (0.7-ps response time). During the course of a typical time-domain experiment, the frequency of the diode laser is locked to the center of a given OCS or N 2 0 absorption line. Temporal changes in the transmitted intensity of the IR probe beam after the photolysis pulse are detected with a cooled (77 K) infrared detector. The time-domain signalsare digitized and averaged on a LeCroy 9400 digital oscilloscope and sent to a personal computer (PC)for storage and later analysis. Transient absorption line shapes are obtained by acquiring numerous (30-50) time-domain signals (each at a separate frequency) in the region of a given OCS or N20 absorption line. This is accomplished by locking the frequency of the diode laser to the peak of a Fabry-Perot etalon fringe produced by a confocal etalon inserted into an auxiliary IR beam path and slowly sweeping the fringe over the frequency range of interest. The collected amplitudes of the time-domain signals at any given point in time after the excimer laser pulse (a typical value is r = 720 ns) yield the transient absorption line shape. The observed signal amplitudes are found to decrease as a function of the number of excimer laser shots due to the buildup of photoproducts on the cell windows and the loss of I2 in the cell. Since static gas mixtures are employed, the recombination or reaction of iodine atoms and ions on the wall of the sample cell following excimer laser MPI can lead to loss of I2 in the cell. In order to minimize errors associated with these problems, the following procedure is employed to determine the rotational distributions. (1) The transient absorption signal is measured on the absorption line of interest. (2) The diode laser is tuned to an "indicator" line and its transient absorption signal measured. (3) The cell is pumpcd out and the diode laser tuned to the next absorptionlineofinterest. TheOCS(1000, J = 19) -OCS(2000, J=20)ortheN~O(OOOl,J=18) +N20(0002,J= 19) transitions are used as the indicator lines. Transient absorption signals are also observed when the diode laser is tuned off-resonance (Le., when the diode laser frequency does not correspond to an OCS or N20 absorption line). These signalsare attributed to thermal lensing and/or Schlieren effects. These two effects are similar in that a nonuniform index of refraction in the gas mixture after the photolysis pulse causes the diode laser beam to be deflected slightly. The final result is that the diode laser light will not be properly focused onto the detector and the number of photons hitting the InSb detector chip will thus be decreased; consequently, an "off-resonance" signal is observed. In the case of thermal lensing, the nonuniform index of refraction is caused by localized heating of the gas mixture due to the nonuniform radial intensity distribution of the excimer laser output. On the other hand, Schlieren effectscan be produced by a nonuniform electron density which is created by excimer laser MPI of 12.
-
N 2 0 (mn'p+l; Jkl, V)
+
-
OCS (mn'p; J , V) hv (-4.7 pm) OCS (m+ln'p; Jk 1, V) (diode laser probing) The resolution of the diode laser (0.0003 cm-I) allows almost any rovibrational state of N20 or OCS to be probed without interference from other spectral lines. In addition, time-resolved laser "Doppler spectroscopy" is employed to determine the translational r e d l velocity (V)along thedirection of propagation of the infrared diode laser via a measurement of the N20 and OCS transient IR line widths following a collision. Experimental investigations of e--OCS scattering have been published by Szmytkowskii2 (total cross section), Tronc and Azria" (differential cross sections for elastic scattering and vibrational excitation of the symmetric stretch mode), and Ziesel et al.I4(dissociativeattachment). Theoretical studies of e--0CS scattering have been performed by Lynch et al.15employing the continuum multiple-scattering method. In the caseof OCS, direct scattering proceeds predominantly via the dipole potential.I6With a permanent moment of 0.7 1D, this systemlinks studieson weakly polar (e- + CO) and strongly polar (e- + hydrogen halides) molecules. Lynch et al.Is have calculated the integral elastic electronscattering cross section for OCS and found weak resonances at energies of 1 0 4 0 eV, in addition to a strong resonance located in the W - e V region. The d-wave-dominated shape resonance has been observed at 3.8 eV in the excitation of the "symmetric" (u3) vibrational mode of 0CS.l3 Below 3 eV, a short-lived 2 I I shape resonancei7gives a structureless intense peak at 1.15 eV for OCS scattering. Quantitative measurements of e--N2O scattering have been performed by Zecca et a1.,18 Kauppila et al.,I9 Szmytkowski et and Kwan et alazi(total cross section), MarinkoviC et al.22 (differential cross section for elastic and inelastic scattering), Kubo et al.23(differential cross section for elastic scattering), AndriC and Hallz4 (differential cross section for vibrational excitation), and Rapp and Englander-Golden25 (ionization cross sections). Theoretical studies of e--N20 scattering have been performed by Herzenberg and co-workers,26who used the nuclear impulse approximationto investigateresonant electron-molecule scattering. In the low-energy range, e--N20 scattering is dominated by a well-known 22+shapere~onance26-~~ centered at 2.25 eV. Above an energy of 5.5 eV, a broad feature has been found in the vicinity of 17eV. This broad hump may be due to the presence of multiple, weak, short-lived, overlapping resonances in this energy region.20 Although the locations of the low-lying shape resonances in these two systems have been determined, very little experimental data are available on the vibrationallyinelastic scattering of these two molecules by electron impact. Up to now, the vibrational excitation of OCS has been obtained from low-resolution energy and angular-dependent studies of differential cross sections by Ehrhardt and co-workers.16The differentialvibrational excitation cross section of N20 by electrons in the energy range 1 4 eV has been studied by Schulz and c o - w o r k e r ~using ~ ~ ~a ~crossed ~ beam apparatus with an energy resolution of 22 meV (-185 cm-1). However, these electron energy loss experimental techniques are not able to unambiguously resolve the 1000, 0200, and 0220 vibrational levels or the rotational states and velocity recoil profiles
II.
Experimental Section
Rovibrational Excitation of NzO and OCS
The Journal of Physical Chemistry, Vol. 97, No. 4, 1993 883 N20(00°1)
N20(1O00)
I
l t
N 2 0 (lOoO, J = 15)
-'t 0
0
4
8
12
Time (we@
-
-
The off-resonance signals are decreased in amplitude relative to the "on-resonance" signals by using gas mixtures with a low I2 mole fraction and by careful alignment of the excimer and diode lasers. After realigning the laser beams, if the off-resonance signal is marethan 5%oftheon-reaonancesignal,theoff-resonance signal is subtracted from the on-resonance signal. The offresonancesignal can beobtained by tuning the diode laser -0.03 cm-1 away from theabsorption line so that thediodelaser intensity does not change appreciably. The OCS (Matheson, 97.5%) and N20 (Matheson, 99.99%) are repeatedly freeze-pumpthawed at liquid nitrogen temperatures and vacuum distilled into Pyrex bulbs before use. The I2 crystals (Aldrich, 99.999%) are placed in a gas bulb which is pumped for several hours before use. I2/OCS and Iz/NzO are made by consecutively flowing each gas into the sample cell. Before filling the cell with an I2/OCS or I2/N20 gas mixture, the cell is seasoned with -200 mTorr of I2 for an hour. Sample pressures in the cell are monitored with a capacitance manometer (range: 0-1 Torr). Samples of pure N20 and OCS are irradiated with the excimer laser to test for the presence of spurious signals. No transient IR absorption signalsareobserved for thevibrational levels investigated here in the absence of 12. the electron precursor gas.
III. Readts
A. N& Rodbrrti0ll.lExcitation. Transient absorption signals are obtained while monitoring the following N20 transitions: 00'1
-
\
I
16
Typical time-resolved N2O infrared absorption signals. The excimer pulse O C C U ~at r = 0. A 1/10 mixture of I2/N20 is used at a total pressure of 27.5 mTorr. (a, top) N20(0001 0002 R(18)) line. (b, bottom) N20(10% 1001 R(1S)) line. Figure 2.
I
00'2 R(7,12,18,24,31,35,40,45,48,52)
Typical signals, shown in Figure 2a, exhibit an initial fast rise (-0.7 ps) followed by a slow decay. The time between N20/ N20 or I2/N20 collisions at the gas density used is -4 ps, much slower than the detector response time. Thus, the fast rise amplitude reflects the increase in population due to direct excitation of N20 by collisions with hot electrons. The slow component of the signal arises from diffusion of the excited N2O molecules out of the diode laser beam path. Similar experiments are performed on the following lines
J(J+l)
Figure 3. (a, top) Boltzmann plot (open squares) for the N~O(0001) rotational distribution observed following collisions with electrons from MPI of 12. The N20(0001) state has a rotational temperature of 296 A 30 K (solid line). (b, bottom) Boltzmann plot (solid squares) for the N20( 1OOO) rotational distribution obtained following collisions with electrons. The N20( 1OOO) state has a rotational temperature of 300 30 K (solid line).
-
probing the 1000 low-frequency stretching vibration: 10'0 10'1 R(7,11,l5,20,26,31,36,41,45,51) Figure 2b shows typical 1000 signals. Like the OOO1 signals,these traces show a prompt increase in absorption followed by a slow relaxation. The fast rise amplitudes are normalized by the excimer and diode laser powers and the fast rise amplitude of the N2O 'indicator" line to obtain the nascent rotational distribution due to electron collisional excitation. The resulting nascent rotational distribution is shown in Figure 3a for the 0001 vibrational state. The distribution peaks at J = 18, very similar to the ambient N20 Boltzmann distribution at 300 K, which peaks at J 15. Similar experiments on the 1000vibrational state produced the rotational distribution shown in Figure 3b, which peaks at J = 15. By fitting both distributions to a Boltzmann temperature profile, the resulting nascent rotational temperatures obtained are Tmt = 296 f 30 K for OOO1 and Trot= 300 f 30 K for 1oOO. The nascent transient line widths of several NzO(0001) and N20( 1oOO)states are measured followingcollisionswith electrons. In all cases, the line shapes are well fit to a Gaussian function. One such Doppler profile is shown in Figure 4 for 0001 0002 R(18), along with a best fit to the Gaussian line-shape profile. The measured line widths for both vibrational states correspond well to room-temperature Doppler widths for N20. The number of excited NzO(0001, J = 18) molecules produced from excimer laser multiphoton ionization of I2 is determined from a diode laser absorption measurement and the N20(0001, R(18))linestrengthtobe(1.2f0.4) X 10I"r3. Thenumber of UV photons absorbed is determined from an excimer absorption measurement to be 4.1 X lOI3 ~ m - ~ The . ratio of the number of excited NzO(0001, J = 18) molecules to the number of photons absorbed is found to be (3 f 1) X lC3. Assuming that the OoOl rotational distribution can be characterized by a room-temperature Boltzmann distribution, the ratio of the number of excited N20(0001) molecules to the number of photons absorbed is (8.0 2.4) X 10-2 for an excimer fluence of -45 mJ cm-2. Similarly, the number of excited N20(1000) molecules to the number of
-
*
Zhu and Flynn
884 The Journal of Physical Chemisrry, Vol. 97, No. 4, 1993 N 2 0 ( 0 0 0 1 , J=18)
-.0.5
+
0.3 I
hv + N 2 0 ( 0 0 0 2 , J=19) I
4
I
4
\
I
a
!
-0.1 -0.010
I
1
I
-0.005
0.000 AV
(cm-')
I
t
I
I
0.005
0.010
-
Figure 4. Nascent Doppler profile of the N20(00°1 00°2 R( 18)) line after collisions with electrons produced from MPI of I2 at 193 nm. The open squares are the data points, and the curve is the best fit to a Gaussian function. The fitted full width at half-maximum is 0.0040 cm-I. For comparison, N 2 0 has a line width of 0.0041 cm-' at room temperature. The small nonzero baseline is an artifact caused by an off-resonance thermal lens signal due to laser heating of the gas mixture.
photons absorbed is (7.6 f 2.3) X for an excimer fluence of -45 mJ cm-2. The dependence of the N20(00°1) and NzO(1000) signals on excimer laser power is obtained by varying the amount of 12 gas in a 43-cm cell inserted in the excimer laser beam path. Varying the excimer power through the use of an absorbing gas has the advantage that it does not change the photolysisbeam alignment. The pressure in the small cell is varied from 0 to 180 mTorr, allowing the excimer power to be varied by a factor of 10. Assuming the fast rise signal amplitude depends on excimer intensity as P,the excimer laser power dependence for the N20(0001) state is found to be n = 2.5 f 0.3 for excimer fluences of 3-30 mJ cm-2. Similarly, a power dependence of n = 2.5 f 0.3 has been obtained for exciting the NzO(1000) low-frequency stretch state. Because of the multimode nature of the laser, the power dependenceof the signals must be consideredas qualitative. Time-resolved signals obtained while monitoring specific rotational states of the 0200 and 0220 levels of N20 are shown in Figure 5 . For these vibrational states, the observed rotational distributions are well fit by -300 K Boltzmann distributions. For the 01 '0vibrational level, a full rotational distribution could not be obtained due to the strong ambient N20 absorption in the cell. The electron density is determined by employing the magnetic flux exclusion principle.1°~*+3' The electron density determined from the flux measurements is 5 X 1O1O for a 5-mTorr I2 sample in a 13.5-cm cell following 193-nm photolysis pulse. The ratio of the number of electrons produced to the number of 193nm photons absorbed is 0.06%. It should be noted that the flux measurementunderestimatesthe electrondensity by about a factor of 10 (ref 10) since the circuit used to detect the flux change has a somewhat slow response time (-2 rs). Thus, any electron/ positive ion recombination or electron attachment to I2 which takes place rapidly will lead to an underestimate of the electron density. As a result, the ratio of the number of electronsproduced to the number of 193-nm photons absorbed will be affected correspondingly. B. Relative NsOVibrational Excitation Cross Sections. The population for each vibrationallevel can be obtained by measuring the corresponding rotational distributions and integrating over all the rotational states. In the present case, the rotational distributions for all excited levels are found to be the same and can be described by a rotational temperature very close to 300 K. As a result, cross-section measurements need be made on
Time ( p e c ) Figures. Typical time-resolved infrared absorption signals. The excimer pulse occurs at t = 0. A 1/10 mixture of I2/N20 is used at a total pressure of 27.5 mTorr. (a, top) N20(02% 02O1 R(27)) line. (b,
bottom) N20(0220
-
0221 R(27)C) line.
-
TABLE I: Experimentally Determined Probabilities for Excitation of NzO by Hot Electrons Which Are Roduced from Excimer Laser Photolysis of 12 vibrational excitation process e-* e-* e-* e-*
--
+ N20(00%) -e- + N20(0001) + N20(00%) e- + N20("1000")(upper)' + N20(00%) e- + N20('0200")(10wer)~ + N20(0000) e- + N20(0220)
probability 1 1.1
0.3 10.08
The "1000" and '02°0" symbols refer to the upper and lower Fermi mixed symmetric stretch/overtone bend states, respectively.
onlyoneor two rotational levels for eachvibrationalstate. Relative cross-sectionmeasurements are thus made on closely spaced W l , 1000,0200, and 0220 lines. In this way, experimental problems arising from k c o n d modes" or excess noise can be reduced. The relative excitation cross sections thus obtained for the OOO1,1~, 0200, and 0220vibrational states of N 2 0 are shown in Table I. In N20, the symmetric stretch (1000) and bending modes (0200) are weakly coupled via Fermi resonance. The relative excitation cross section between the upper and lower Fermi mixed levels has been determined to be ?r~~,y,,~/.rru0200~ = 3.5 f 0.8, where ?rutis the cross section for excitation of state i by electron-molecule scattering. The excitation cross section for 1000 relative to 0001 is found to be T U ~ O ~ ~ / T U=W1.1O f~ 0.3. ~ Due to the small signal amplitude for the 0220 states and the comparable magnitude betweenon-resonanceandoff-resonancesignah,an accuratecrosssection measurement for this state is difficult to obtain. Here, only an upper limit is given. Considering the double-vibrational degeneracy for the bending modes of N20,the upper limit for the 0220cross section is found to be ?rao2~,,2/?ru~ol S 0.08 f 0.04. C. OCS RovibrationalExcitation. Immediately after the 193nm multiphoton ionization of iodine in an I2/0CS mixture, transient absorption signals are observed while monitoring the following OCS transitions:
-
10'0 20'0 R(6,8,15,20,30,41,47,55) All signals exhibit a detector-limited fast rise followed by a more slowly changing component,as shown in Figure 6. At the pressure used (27.5 mTorr), the mean collision time between the ambient gas molecules is -4 MS, much slower than the detector response time. Thus, the fast component of the signals is due to nascent
Rovibrational Excitation of N20 and OCS
The Journal of Physical Chemistry, Vol. 97, No. 4, 1993 885 OCS(lOoO, J-31)
I I
.
I
'
0
I
'
I
'
8
4
I
12
+
hv + OCS(2O00, J=32)
'
16
!
Time ( p s e c )
Figure 6. Typical time-resolved OCS infrared absorption signal. The excimer pulse occurs at t = 0. A 1/ 10 mixture of I2/0CS is used at a total pressure of 27.5 mTorr. The absorption line probed is 10°O 20°0 R( 19) of OCS.
-
-+ .-
0.0 -0.010
I
I
I
I
-0,005
0.000
0.005
0.010
AV
(cm.')
-
Figure 8. Nascent Doppler profile of the OCS( 10% 2000 R ( 3 1)) line after collisions with electrons produced from MPI of 12. The open circles are the data points, and the curve is the best fit to a Gaussian function. The fitted full width at half-maximum is 0.0036 cm-1. The ambient room-temperature OCS line width is 0.0033 cm-I. The small nonzero baseline is an artifact caused by an off-resonance thermal lens signal due to laser heating of the gas mixture.
.
7
z
OCS (02'0, J = 40)
-2
5. A
0
1000
2000
3000
4000
0.6
J(J+l)
Figure 1. Boltzmann plot for the OCS( 10%) rotational distribution obtained illowing collisions with electrons produced from MPI of 12. The OCS( 10%) state has a rotational temperature of 274 f 30 K (solid line).
rovibrational excitation of OCS by hot electrons. The slower component is due to the diffusion of the OCS out of the diode laser probe beam. The fast rise amplitudes are normalized by excimer and diode laser powers and the fast rise amplitude of the 'indicator" line to obtain the relative populations for the different rotational states. The resulting nascent rotational distribution is shown in Figure 7 for electron-scattering excitation of OCS( 1oOO). The nascent 1000 rotational distribution is wellcharacterized by a Boltzmann distribution of 274 f 30 K. The nascent transient line widths of several OCS( 1oOO) lines were also measured. In all cases, the line shapes are well fit to a Gaussian function. One such Doppler profile is shown in Figure 8 for the OCS( 10% 2000 A(3 1)) line, along with a best fit to a Gaussian line-shape profile. All the measured line widths correspond to room-temperature ambient Doppler line widths for OCS within experimental error. The number of excited OCS( loOO, J = 19) molecules produced following excimer laser multiphoton ionizationof I2 is determined from diode laser absorption signals and the OCS line strength measurements to be (1.5 f 0.5) X 10" The number of photons absorbed is determined from excimer laser absorption measurements to be 3.5 X 10" cm-3. The ratio of the number of excited OCS( 1000, J = 19) moleculesto the number of photons absorbed is found to be (4.3 f 1.4) X 10" for an excimer fluence of -45 mJ cm-2. Assuming that the loo0 rotational distribution can be characterized by a room-temperature Boltzmann distribution, the ratio of the number of excited OCS( 10°O) molecules to the number of photons absorbed is 0.16 f 0.05 for an excimer fluence of -45 mJ The dependence of the OCS(lOO0) signal on excimer laser power is obtained by varying the amount of 12 gas in a 43-cm cell inserted in the photolysis beam path. Assuming the fast rise
-
:[ OCS (OOol,J = 39)
0.0
p Time (pec)
Figure 9. Typical time-resolved OCS infrared absorption signals. The excimer pulse occurs at t = 0. A 1/10 mixture of I2/OCS is used a t a total pressure of 27.5 mTorr. (a, top) 0200 12% R ( 4 0 ) line. (b, 1001 R ( 3 9 ) line. bottom) OOol
-
-
amplitude of the diode laser signal is proportional to the excimer intensity to the nth power (signal a In), the excimer laser power dependence for the OCS( 1oOO) state is found to be n = 2.4 f 0.3 for excimer fluences of 3-30 mJ D. Relative OCS Vibrational Excitation Cross Sections. Transient absorption signals obtained while monitoring various rotational states of the Wl and 0200 vibrational levels of OCS are shown in Figure 9. Typical signals exhibit a detector-limited fast rise followed by a slow decay. Fast rise amplitudes are normalized by excimer and diode laser powers and the amplitude of the indicator line to obtain the nascent rotational distributions due to electron inelastic scattering. The 0200 distribution can be characterized by a 31 1 i 30 K Boltzmann distribution, while the 0001 states fit a room-temperature (300 f 30 K) Boltzmann distribution. The transient line widths for therovibrationalstates probed are well fit to a Gaussian line shape and can be characterized by an OCS room-temperature Doppler width.
Zhu and Flynn
886 The Journal of Physical Chemistry, Vol. 97, No. 4, 1993
TABLEII: Ex meatall Detewiaedhobnbilitiesfor Excitation of by Hot le~trolrpWhich Are Produced from Excimer Laser Photolysis of I2
&
E
--
vibrational excitation process e-+ + OCS(ooo0) e-* OCS(ooo0) e-* + OCS(ooo0)
+
a
-
eee-
+ OCS( 1000) + 0CS("0200")(uppera) + OCS("WJ1")(lower)O
probability 1 0.6 0.14
The '0200" and "W1"symbols refer to the upper and lower Fermi
mixed overtone bend/symmetric stretch states, respectively, for OCS.
Detection of the low-lying 01 '0state is difficult due to the strong absorption of ambient OCS gas. The population for each vibrational level can be obtained by integrating over the corresponding rotational distributions. Since the rotational distribution is similar for all the vibrational levels, the relative vibrational populations can be determined by measuring the population of only one rotational state for each vibrational level. These two methods yield similar relative number densities within experimental error. Assuming the cooling rate of the electronsbelow the threshold for OCS vibrationalexcitation is the same for differentvibrational modes? the relativeexcitation cross sections obtained for the lOOO, OOO1, and 0200 vibrational states of OCS are shown in Table 11. The ratio of the cross section between the upper Fermi mixed level (0200) vs the lower energy Fermi level ( W 1 ) is determined to be ~a02~~2/xuoo01~ = 4.4 i 1.5. The excitation cross section for 0200 relative to 10% is found to be ~ U O ~ C Q ~= / 0.59 ~ Uf I 0.13. ~ ~
IV. Discussion A. Simple Kinematic Considerations. The experimental data indicate that rotational and translational excitation of N 2 0 and OCS by electronsis quite weak, consistentwith earlier observations on electron402 scattering.*JOThe negligible linear momentum transfer to N 2 0 and OCS produced by electron-N20 and electron-OCS collisionsis in good agreement with a simple billiard ball, elastic collision model.7332The average energy transferred in such a model is
AE
[2MeMm,I/(Me + M ~ ~ J ~ I ( E O - E v i J where MmO1 is the mass of N20 or OCS molecules, Meis the mass of the electron, Evibis the energy in cm-1 transferred to vibration in the collision process, and EOis the translational energy of the hot electrons produced by the excimer laser photolysis. The total averagetranslationalenergy after an electron/NzO or an electron/ OCS encounter is
E, = (3/2)kT
+ AE
where T is the temperature of the gas mixture before excimer laser photolysis. The calculated line widths are assumed to scale as (2E~/3k7')I/~ times the room-temperature Doppler width of N20 or OCS (0.00416 and 0.00330 cm-I, respectively). Assuming the electrons have 1-3 eV of translational energy, the billiard ball model calculation predicts a negligible increase (-0.0000014.000003 cm-I) in the transient N2O or OCS line widths. The negligible amount of linear momentum transfer observed in the present experiments is largely a consequence of the large mass mismatch between the collision partners in the electron-N20 and electron-OCS systems. The lack of rotational energy transfer is also consistent with an electron-N20 and electron-OCS scattering process. The maximum rotational energy transfer from an electron to an N20 or OCS molecule can be calculated from the total angular momentum L available to the electron-N20 and electron-OCS system for backward scattering33
L = b(2mV) where b is the impact parameter, m and Vare, respectively, the
mass and velocity of the electron, and 2mV is the momentum transferred to the N20 or OCS molecule under the conditions of backward scattering. For OCS, the C - 0 and C-6 bonds have lengths of 1-15and 1.56A, respectively; the center of mass is 0.52 A from the C atom shifted toward the S atom; and the relative electron-OCS velocity is V = 8.4 X lo7 cm/s (2 eV). The maximum total angular momentum available to the system is calculated from these parameters to be 3.9 h/2x when hitting the oxygen end and 2.4 h / 2 r when hitting the sulfur end of OCS. This corresponds to a AJ of -2-4, in agreement with the experimentally determined OCS rotational distribution. B. Dependence of Signal Amplitudes on Excimer Laser Power. The power dependence of the N20(0001), N20(1000), and OCS( 1OOO) fast rise signal amplitudes indicates that the species which vibrationally excites N20 and OCS must be produced by a two or more photon process. This is consistent with the steps I, I,(D)
-
+ hv(193 nm) + nhv( 193 nm)
I,(D)
I,+ + e-
At 193 nm, I2 absorbs very strongly, mainly to the Y = 149 level of the D state.34 12(D) thus formed will either absorb a second photon or fluoresce back to the ground state or to the low-lying electronically excited a' state in 15.5 ns.35 Since the vertical ionization potential of I2 is 9.3 eV,36 the absorption of two or more 193-nm (6.4-eV) photons will put 1 2 well above the ionization threshold. Therefore, the power dependence measurements are consistent with the picture of electrons as the major excitation species. In addition, the presence of electrons has been verified by the time-resolved magnetic induction technique.IOJ9-31 C. Relative Vibrational Excitation probrbilities. Similar vibrational excitation probabilities have been obtained for the OOO1 and loo0 vibrational states of N 2 0following collisions with electrons produced from MPI of 12. This experimental result is in accord with the low-resolutionstudies of e--N20 scattering by Schulz and co-w0rkers.2~ Past studies have shown that e--N20 scattering exhibits a Q+ shape resonance at -2.3 eV.27 The vibrational modes excited belong to four series, the n00, n10, nO1, and n02 series with the quantum number n ranging from 0 to 7. At the center of the Q+resonance, the no0 series is most intensely excited and has a cross section of (1.2 f 0.4) X cm2 for n = 1 and an exponentiallydecreasing magnitude for successively higher n. On the other hand, below the energy range of the 2Z+ resonance (Le., near 1 eV), only the fundamental levels 0110, 1oo0, and oOO1 have an appreciable excitation cross section. The cross section for excitation of the OOO1 level stands out as the largest, probably as the result of a direct excitation process due to the large transition dipole moment associated with the OOO1 ~tate.2~ Since the precise electron energy resulting from MPI of iodine in the present study is not known, the experimental results obtained here most likely have contributions both from the 2Z+ resonance and from direct scattering processes. However, the ratio of the cross sections for excitation of the two stretch states agrees well with the results from low-resolutionstudies by Schulz and co-workers at an incident energy of lcss than 2.3 eV. The high-frequency stretch (1oo0) state of OCS is found to be significantlymore populated than thelow-frequencystretch(OOO1) state following collisions with electrons produced from MPI of 12. This present experimental result is in agreement with a direct scattering process. In the case of OCS, direct scattering is dominated by the dipolGion charge interaction potential. Direct vibrational excitation is thus expected to scale like the corresponding dipole transition matrix elements squared.I6 For OCS, these matrix elements squared are (0.134 au)* and (0.0268 au)2, respectively, for the high- and low-frequency stretches.)' Since the dipole transition matrix element is a good deal larger for the high-frequency stretch than for the low-frequency stretch level,
-
Rovibrational Excitation of NzO and OCS the high-frequency 1oO0 stretch state of OCS is expected to be more highly excited by direct e--0CS scattering. The observed ratio for the excitation of loo0 vs W 1 in OCS is 7 (see Table II), while the ratioof the dipole transition matrix elementssquared is25. Thissuggeststhat the lOOOand W l excitationprobabilities cannot be due entirely to the dipoleion charge interaction. Excitation of OCS by electrons also exhibits a ll jesonance at 1.15-eV collision energy, which strongly excites the bending mode. The present results for vibrational excitation of lo00 and W 1 in OCS may be affected by this resonance,although direct scattering seems to be the dominant mechanism for producing these levels. The number of excited NzO(W1) molecules per photon absorbed was determined to be 0.08,while the number of excited OCS(loO0) molecules per photon absorbed is 0.16. Thus, the vibrational excitation probability for exciting the high-frequency stretch is larger for e - 4 C S scatteringthan for e--NzO scattering. This result is in agreement with past studiesof e--0CS scattering where e--OCS total scattering cross sections are found to be noticeably larger than those for N2, N20, and C02. D. Quantum Interference Effects. An interesting feature of this study is that the upper energy state of the Fermi doublets is significantly more populated via electron scattering than the lower energy state for both N 2 0and OCS. The observed results can be well explained in terms of a quantum mechanical interference in the electron-scattering experiments first reported for e - 4 0 2 scatterings9 Unlike COZ, where, in the harmonic oscillator limit, uI is nearly equal in energy to 2uz and there is a strong mixing between the 11oOO) and 10200) harmonic oscillator states, Fermi resonance is weaker in N20 and OCS. Therefore, the quantum interference effects are much reduced because the upper and lower energy states are not equal mixtures of 11OOO) and 10200) for these two molecules. As a result, the ratio of the excitation probability for the upper energy member compared to the lower energy member of the Fermi doublet is only about 4, compared to about 10 for C02. It is particularly interesting to look at OCS where the upper energy state is significantly more populated than the lower state despite the fact that it has about 90% 0200 character. The collisional excitation probabilities P gand ~ Pg? from the ground vibrational state to the Fermi coupled vibrational levels is proportional to 1(oooO1Vlq)12 or to
P,,
-
[sin e(oooolvliooo)+ cos e(o0~o~y02~0~~~ (quupper energy state)
pgL lcos e (ooool
ooo) - sin e (oo0ol~lo2Oo)1' (qLlower energy state)
where V defines the interaction potential and 8 values are approximately 7 1 and 14O, respectively, for NzO and OCS. From the ratio of the excitation probability between the "1oOO" and "0200" levels, we obtain (oooOlVlloO0) = l.O(oooO)y10200) for e--N20 scattering and (oooO(qW1) = 0.8(OOOOl~0200)for e--OCS scattering, suggesting almost equal probabilities for exciting one quantum of "symmetric stretch" motion and two quanta of bending motion in the electron-scattering experiments. (This analysis assumes the two matrix elements have the same sign. If they are allowed to have opposite signs, the results are (ooOoJ~lO00)= 44(ooOo)y10200) for N20 and (oooO~~loOO) = -0.2(ooOo(~O200)for OCS.)
v.
coaclurions
(1) High-resolutioninfrareddiode laser absorptionspectroscopy has been employed to determine the nascent rotational distributions and transient line widths in a number of N20 and OCS vibrational states following collisions with electrons produced from 193-nm excimer laser multiphoton ionization of 12.
The Journal of Physical Chemistry, Vol. 97, No. 4, 1993 887 (2) Although a significant amount of vibrational excitation has been observed in N 2 0and OCS, very little energyis transferred to the rotational and translational degrees of freedom of these molecules due to electron collisions. The lack of translational and rotational excitation is in good agreement with a simple kinematic picture of collisionsbetween light and moderate energy electrons and heavy molecules. (3) The relative probabilities for collisional excitation of N 2 0 vibrational states observed in this study are W 1 : 1ooO.0200:0220 1:1:1:0.3:
