J . Phys. Chem. 1993, 97, 7242-7246
7242
State-Resolved Photodissociation of N20 Thomas F. Hanisco and Andrew C . Kummel' Department of Chemistry, University of California, Sun Diego, Lo Jolla, California, 92093 Received: March 2. 1993
The photodissociation of N2O at 203-205 nm has been investigated using state-resolved resonance-enhanced multiphoton ionization. The data are consistent with one-photon dissociation occurring via the bent 2'A'(B1A) excited state. The dissociation probability increases substantially over a nozzle temperature range 300-1 500 K. The N 2 fragment is highly rotationally excited as a result of the excited-state geometry. The rotational distribution of the N2 fragment is peaked at 1.4 eV and has a width of 0.6 eV. The average rotational energy of the N2 product is 1.4 f 0.1 eV; the average translational energy is 0.38 f 0.04 eV. There is no observed vibrational excitation in the N2 product. The O(*D) product has an average translational energy of 0.67 f 0.06 eV. The rotational energy of the products accounts for 57 f 4% and the translational energy accounts for 43 f 4% of the energy available to the products.
I. Introduction The photodissociation of nitrous oxide near 200 nm produces ground-state (X'Z;) nitrogen molecules and excited oxygen atoms according to the mechanism' N20
+ hv
-
N2(0,J)
+ O*
(1)
The oxygen product is produced predominantly in the excited lD state rather than the excited lS state or the 3P ground state for dissociation wavelengths between 185 and 230 nm.1-4 Thus, the photodissociation of N2O is useful for producing excited oxygen atoms for hot-atom chemistry.596 However, the complete understanding of the dissociation of N2O has been limited because of the lack of information on the nitrogen product internal states. A complete understanding of the photodissociation of N20 requires knowledge of the electronic, vibrational, and rotational properties of both photofragments. The internal state distributions of diatomic molecule photofragments from the photodissociationof triatomic molecules have been measured for a variety of triatomic molecule^.^.^ The photodissociationof a triatomic molecule (with no internal energy) into atomic and molecular products follows the energy conservation equation
hv = Do+ E ,
+ E, + E, + E,
(2)
where hv is the energy of one or more photons, DOis the dissociation energy of the molecule into the product fragments, and EE,ET, Ev, and ER are the electronic, translational, vibrational, and rotational energies of the product fragments. Since the electronic properties of photofragments are usually well-known, the measurement of the disposition of energy into translation, vibration, and rotation of the products is important to complete the understanding of the photodissociation process. The translational energy distributions and the anisotropy of the fragment velocity and angular distributions of the N2 and O(1D) photofragments have been measured by both time-offlight9 and Doppler profile ~pectroscopy.~J0The anisotropy of the angular distribution of the fragments with respect to the polarization direction of the photolysis laser is described by
q e ) = [i + BP,(cOS e ) l p T (3) where P2 is the second LeGendre polynomial and 0 is the anisotropy parameter." Felder, Haas, and Huberg measured the translational energy and angular distributions of the N2 and 0 fragments for dissociation at 193 nm with time-of-flight techniques. They determined an average translational energy of 0022-3654/93/2097-7242t04.00/0
1.16 eV (42% of the available energy, Ea", at 193 nm) and an anisotropy parameter, j3 = 0.48 f 0.02. Springsteen et al.1° measured the Doppler profile of the O(lD) fragment using vacuum ultraviolet laser induced fluorescence. They measured /3 = 0.50 i 0.05 and an average translational energy of 1.18 eV (42% of Ea") for dissociation at 193 nm. Though these two measurements are in excellent agreement, measurements by Shafer et ~ 1 are. ~ not. Shafer et a1.4 measured the Doppler profile of the O(lD) fragment at 205.47 nm using multiphoton ionization in a singlelaser experiment. They determined /3 = 2andanaveragefragment translational energy of 2.32 eV (96% of Ea") for dissociation at 205.47 nm. The values of Shafer et al. are based in part of the assumption that the amount of internal energy of the N2 fragment is small, since the fits to the Doppler profile are not unambiguous when only one laser (and hence only one polarization) is used. The sharp disagreement of the results of Felder et al. and Springsteen et al. with Shafer et al. emphasizes the importance of determining the internal energy of the photofragments. Sivakumar et ~1.12913measured the rotational and vibrational distributions of the CO photofragment from the dissociation of OCS, which has a linear XlZ+ ground state like N2O. Their results show a dramatic rotational state population inversion centered at J = 56 for dissociation at 222 nm. These authors attribute the high degree of rotational excitation to dissociation via a bent excited state. Miller et a1.14 and Stolow and Lee15 measured the photodissociation of C02, which is isoelectronic with N20. The measurements of these authors were consistent with excitation to a bent excited state as well. The dissociation of N20 can also occur via a bent excited state. For example, calculations by Hopper16 show that several bent excited states correlate with X'Z; nitrogen and lD2 oxygen products from dissociation, notably the excited states: 2'A'at 5.3 eV and 11A" at 5.7 eVabove theN20(X12+,0,0,0) groundstatewithaN-N-O bond angle of 130°. Several authors have measured the absorption probability of NzO versus temperature and suggest that the increased absorption found with increased temperature is due to the higher population of the bending vibration at elevated temperatures.17-19 In this report, we present the results of the state-resolved photodissociation of N2O at 203-205 nm using resonanceenhanced multiphoton ionization (REMPI).The results include measurementsof (1) therotationaldistributionof theNzproducts, (2) the dependence of the dissociation probability on temperature and laser power, and (3) the angular dependence of the photofragments. The results are discussed in terms of theexcitedstate symmetry and the energetics of the dissociation. 0 1993 American Chemical Society
State-Resolved Photodissociation of N 2 0
The Journal of Physical Chemistry, Vol. 97,No. 28, 1993 1243
)-
‘“t1
....’
___,
tX
N, a”
IZf(0,O) Q Branch
140
Molecular beam ..............
J =40
60
50
90
80
70
100
120
X
Figure 1. Experimental geometry used for the photodissociation of N20 and REMPI of O(lD) and Nz. The axes of the molecular beam, laser,
and TOFMS are mutually orthogonal. The molecular beam propagates along the y-axis, the laser propagates into the page along the x-axis, and the ions arc extracted by the TOFMS in the z direction. 11. Experimental Section
The photolysis of N2O was performed with the same molecular beam source and tunable laser source described in detail elsewhere.20*21A molecular beam of N20 (Matheson ultra high purity, 99.99% min) was generated by expanding 3 bar of a 15% N 2 0 / H e mixture through a pulsed valve nozzle with a 2.0-mm orifice. A resistively heated S i c tube attached directly to the output of the nozzle allows heating of the molecular beam between 300 and 1500 K.22 The molecular beam passes through a 0.49mm conical skimmer, a mechanical chopper, and a 0.3-cm collimator before entering the UHV chamber, which has a base pressure of 1 X 1@l0 Torr. The rotational temperature of the N20 beam cannot be measured directly, but the rotational temperature of N2 expanded under identical conditions is less than 30 K. A single tunable laser source was used to photodissociate N20 and to detect the N2 and 0 photofragments at 204 nm. The N2O molecules were dissociated with approximately 1 mJ of 100% horizontally polarized light focused with a 10-in. fused-silica lens to the center of the molecular beam. The laser power was monitored with a pyroelectricdetector (Laser Precision Corp. RJ-735)at the exit window of the detection chamber. In addition, the laser power could be changed over the range 200500 pJ with a variable attenuator (Newport 535-9, and the polarization could be rotated with a half wave-plate. The detection of the Nz and 0 photofragments was accomplished using resonantly enhanced multiphoton ionization (REMPI) and time-of-flight mass spectroscopy (TOFMS). The Nz products were ionized with two-photon REMPI using the a”‘Zi X’Z; transition first reported by Lykke and Kay.23,24 The spectroscopic analysis used for identifying transitions and determining rotational state populations is reported elsewhere.20 The O(lD2) products were ionized with two-photon REMPI of the ‘PI-ID~and lF3--ID2 transitions reported by Pratt, Dehmer, and Dehmer.25 The ions were extracted with the time-of-flight detector which was positioned orthogonal to the plane defined by the molecular beam and laser propagation direction, as shown in Figure 1. The ions are detected with a multichannel plate and counted with a gated integrator.
202.4
202.6
-
202.8
203.0
20 2
203 4
203 6
III. Results The rotational spectrum of the N ~ ( U ” =0,J) product from the photodissociation of NzO is shown in Figure 2. The wavelength corresponds to both photodissociation and detection wavelengths. The spectrum exhibits the 2:l even-odd J-state intensity ratio characteristic of N2, verifying that Nz is the primary product from dissociation. We note that N O absorbs strongly near 205 nm in a one-photon resonant ionization process, but N O is not observed between 202 and 206 nm. The multiple peaks above
2040
Figure 2. a/’lZ; X’Z’(0,O) transition Q branch rotational spectrum of the N2(o”= 0,J)prduct from the photodissociation of N20.
I I
II
II
II
I
40
45
50
55
- N,O Photodissocialion N, Inelastic Scaftenng
20
=B
-
I
15
P
10
il
5
I b’
-
-
203 8
Wavelength (nm)
20250
20255
20260
20265
20270
Wavelength (nm)
20275
20280
Figure 3. Rotational spectra of N2 scattered from a W surface at 4 eV (dashed line) and of the N2 product from N20 photodissociation.
203.2 nm are probably due to perturbations in the N2 a” state, since the positions of the peaks are inconsistent with transitions from u” = 1 of N2. Using the spectroscopic constants reported previously,20we predict that the ut’= 1, J = 75 transition occurs somewhere above 203.8nm, though the exact position is difficult to determine with only one rotational constant. Some u” = 1 may be present in the spectrum, but the amount of u” = 1 is certainly less than 2%of the u”= 0 product. The rotational lines shown in Figure 2 were identified by comparing the photodissociation spectrum with a spectrum obtained from Nz inelastically scattered from a tungsten surface at 4 eV. Figure 3 shows the
spectraofN2bothfromNzOphotodissociationandfromscattering off a W surface. The overlap between the two spectra verifies that the N2 photofragment spectra is of molecules in u” = 0 and provides an accurate reference for the identification of the rotational lines of the N2 photofragment spectra. The rotational populations for states below J = 77 were determined by integrating the peak intensities for each J state and dividing by thenuclear spin state degeneracy. The populations for J I 77 were determined by averaging several peaks together. For example, the expected wavelength region for J = 77-80 was determined by extrapolating from the positions of the lower J states. The areas of all peaks within this wavelength region were summed together, multiplied by 1.5 (to account for nuclear spin statistics), and divided by 4 to obtain an average. The rotational state populations are shown versus rotational energy in Figure 4. The populations that were determined with averaging are represented by asterisks and the populations determined from
Hanisco and Kummel
7244 The Journal of Physical Chemistry, Vol. 97, No. 28, 1993
*, * ....
Population Rotation Translation NZ
Temperature (K) 1000
,
900
300
1500
I
O(’Q
- 0
02
00 0 1 0
-I
00
05
I 10
I
15
20
25
Energy (eV)
Figure 4. Rotational population, aE(J)(app), versus rotational energy for the Nz photofragment, The solid circles are for populations that were determined from sinale rotational lines and the asterisks correswnd to averaged popu1ations:as explained in the text. The solid line is a Giussian fit to the population distribution. The dash-dot curve corresponds to the total energy availableto translationassuming a Gaussian rotationalenergy distribution. The dashed and dotted curves represent the translational energy distributionsof the N2 and O(*D)products that correspond to the total translational energy distribution.
individual J states are represented by solid circles. The solid line represents a Gaussian fit to the rotational populations. Thedashed and dotted lines correspond to the translational energy distributions of the fragments. The calculation of these distributions is discussed in Section IV B. The peak of the population distribution is at Erot= 1.4 eV (J = 74), and the full-width a t half-maximum of the distribution is approximately 0.6 eV. The average rotational energy of the N2 photofragments is 1.4 f 0.1 eV. The populations were not corrected for any change in the photodissociation probability over the wavelength region examined. The effect of temperature on the dissociation probability was measured by monitoring the O(lD) 1F&Dz transition over a range of nozzle temperatures between T, = 300 and 1500 K. The vibrational population of the molecules in the molecular beam could not be measured directly, but the vibrational temperature of Nz under identical expansion conditions was measured and found to be 10-15% lower than the nozzle temperature. We therefore present the NzO measurements with corresponding uncertainties in the temperature of 10-15%. The bending vibrational mode of N20, v2 = 588.78 cm-l,26 is particularly interesting because of the possible role of the bending vibration in accessing a bent excited state during the dissociation. Between T,, = 300 K and T,, = 1500 K, the population of vibrationally excited molecules in v2 increases from 6% to 56% of the total population. The dependence of the O(lD) REMPI signal on temperature (top axis) and vibrational population, Nv.l/N,,~, of the bending vibration (bottom axis) is shown in Figure 5 . The straight line is drawn through the data points to guide the eye, not as a fit to any function. The dissociation probability increases by a factor of 3.6 between T,, = 300 K and T,, = 1500 K. Similar increases in the absorption probability of N20 a t 205 nm with increasing temperature have been attributed to increased population of the bending vibration at higher temperatures.”-19 The effect of temperature on the rotational distribution of the N 2 photofragment was also measured, but the effect was relatively small. Even a t 1300 K the rotational distribution of the N2 photofragment is only slightly different from the distribution shown in Figure 2. The major difference is that the peak in the distribution is shifted to J = 78 (E,,, = 1.5 eV). This small shift can be explained by the greater amount of energy available to the products after dissociation (see below).
03
04
05
06
Nu=lNu=o Figure 5. REMPI signal of the O(lD) fragment versus the vibrational population,Nu-l/NU-o, of the v2 bending mode (bottom axis) and nozzle temperature (top axis). The error bars represent the uncertainty from three separate measurements. The error in the vibrational population is roughly 10%. 2.4
4
2,2]1
o 0 0
’D n = 3.0 ?r 0.2
N,
n = 3.1
* 0.21
2.0
‘Z
I
l
2.35
o
2.40
2.45
2.50
log(Laser Power)
2.55
2.60
Figure 6. Logarithm of the ion signal versus logarithm of the laser power for O(*D) (open circles) and J = 74 of NZ(closed circles).
The power dependence of the photodissociation process and of the REMPI detection was determined by monitoring the product REMPI signal while varying the laser power. The power dependencies for both the N Zand O(lD) products detected with two-photon REMPI of the a ” l Z i X’Z+(O,O)Q(74) and ~FsID2 transitions are shown in Figure 6. T i e lines are displaced from each other because the O(lD) REMPI signal is roughly 5 times larger than the N ZREMPI signal. A fit to the form Z = P”,where I = the REMPI signal and P = the laser power, yields n = 3.1 f 0.2 for the N2 product (solid circles) and n = 3.0 f 0.2 for the O(lD) product (open circles). The slope of n = 3 indicates that three unsaturated steps are required for the overall process of photodissociating Nz0 and detecting the products with REMPI. In the Nz Z-Z transition, theionization stepis saturated, but resonant steps are not; thus the REMPI signal depends on the square of the laser power.20 The three-photon overall dependence of photodissociation and REMPI indicates that the photodissociation step requires only one photon. Assuming that the resonant steps of the O(1D) IPI-IDz transition are not saturated, the three-photon dependence of the overall photodissociation of Nz0 and REMPI of O(lD) suggests that the oxygen product also results from a one-photon dissociation of N2O. The symmetry character of the photodissociation transition was examined using time-of-flight (TOF) measurements for two different laser polarizations. This technique is similar to the techniques described in detail by Penn et a1.z7and Chu et a1.28 The extraction voltage on the TOFMS was reduced considerably so that the arrival timeof ions with different initial velocity vectors
-
State-Resolved Photodissociation of N20
The Journal of Physical Chemistry, Vol. 97, No. 28, 1993 1245
200 h y1
5.0
5.5
6.0
6.5
7.0
7.5
8.0
‘t
0
400
2
2
e
200
3
loo 0
Y 3.5
4.0
4.5
5.0
5.5
Flight Time (Ps)
6.0
6.5
Figure 7. Time-of-flight distributions for (a) N2 and (b) O(lD)
photofragmentsusing horizontal (open circles)and vertical (solid circles) polarization. could be resolved. Becauseof the geometry of thedetector (Figure l), theTOFis only sensitive toinitialvelocityvectors withdifferent components in the z direction. Figure 7 shows TOF distributions for both (a) N2 and (b) O(lD) using horizontal (open circles) and vertical (solid circles) polarizations of the laser. The flight times shown correspond to the time between the laser pulse and the arrival time of the ions at the TOF detector. For both N2 and O(lD) the TOF distributions show one peak for horizontally polarized light and two peaksfor vertically polarized light. Longer TOF times correspond to molecules or atoms moving away from the TOF detector, and shorter times correspond to molecules or atoms moving toward the TOFdetector. TheseTOF distributions are not calibrated for determining absolute velocities and represent only the relative velocities of the fragments.
IV. Analysis and Discussion A. Excited-StateSymmetry. Ifwe assume that the dissociation recoil occurs along the N-0 bond axis, the symmetry of the dissociation transition can be determined. For one-photon dissociation, a parallel transition results in photofragments with velocity vectors peaked along E,the polarization vector of the laser, in a cos2 8 distribution.11 A perpendicular transition results in velocity vectors peaked in the plane perpendicular to the polarization vector. For horizontally polarized light, a parallel dissociation transition would produce products with velocity vectors along the molecular beam axis (the y-axis in Figure l), resulting in a single T O F peak. A perpendicular dissociation transition (E IF) would produce products with velocity vectors in a plane perpendicular to the plane defined by the molecular beam axis and the laser propagation direction, resulting in a single broad TOF peak. Similarly, for verticolly polarized light, a parallel transition would produce products with velocity vectors peaked perpendicular to the plane defined by the molecular beam and laser propagation direction, resulting in two TOF peaks: an early peak corresponding to fragments initially moving toward the TOFMS and a late peak corresponding to fragments initially moving away from theTOFMS. A perpendicular transition would produce products in the plane defined by the molecular beam and the laser propagation direction, resulting in a single TOF peak.
X’Z+ -
1
Figure 8. Excitation energy relative to the N20(X1Z+,0,0,0)ground state for excited states that correlate with N2(X’Zl) and O(lD) products. The states corresponding to C., (linear) states are shown on the left of the diagram, and the C, (bent) states are shown on the right. The dashed line at 3.64 eV represents the N,(X’ZlO,O) and O(lD) product energy relative toN20(X1Z+,0,0,0). Thevaluesforthe transition energies are from ref 16.
The data shown in Figure 7 indicate that the photofragments recoil along the direction of the electric vector of the laser. In terms of the anisotropy parameter, our results indicate that j3 > 0.” However, since these results are qualitative, we cannot assign a specific value to j3. This result is consistent with the positive values measured for j3 by Felder et a1.,9 Shafer et a1.,4 and Springsteen et al.1° The identical trend for both N2 and O(1D) and the j3 > 0 also suggest that the photofragments are produced from the same excited state and that the transition to this excited state is predominantly parallel. The p = 0.48 and j3 = 0.5 measured by Felder et al. and Springsteen et al., respectively, indicate that either the excited state is mixed, the excited state has a long lifetime, or the recoil direction of the fragments is not exclusively along the axis of the transition dipole moment. However, the single peak in the rotational distribution is consistent with dissociation occurring through a single excited state. If two excited states were involved in the dissociation, we would expect to see a bimodal rotational state distribution, similar to that observed by Sivakumar et al.12J3for the dissociation of OCS at 222 nm. Though the data cannot rule out a long-lived excited state, the large rotational excitation of the N2 products, which is consistent with a bent excited state, strongly suggests that the reduced asymmetry in the fragment angular distribution results from nonaxial recoil of the photofragments. Several excited states of N20 that correlate with the N, (X‘Z;) and O(lD) products are shown in Figure 8. The energy of states for linear N2O (Cmosymmetry) and bent N 2 0 (C, symmetry) relative to the N2O(XlZ+,O,O,O) ground state is shown on the vertices. The dashed line at 3.64eV represents the energy of the photoproducts relative to theNzOground state. Thevalues for the transition energies are from ref 16. The states with C,, symmetry that correlate with the N,(X’Z;) and O(lD) products are: XIZ+,BIA,andCIII. ThetransitiontotheBstateisorbitally forbidden, and Hopper calculates that the B and C states lie at 7.65and 8.35 eV, respectively, outside the range for a one-photon absorption a t 204 nm (6.1 eV). The states with C, symmetry that correlate with the observed photofragments are, with the corresponding Cm,state in parentheses, 1lA’(XIZ+),2lA’(BlA), and llA”(AIZ-). Transitions to the 2IA’ and 1’A” states are both allowed and energetically feasible. Hopper calculates that the 21A‘ and 11A” states are 5.3 and 5.7 eV above the ground state with a N-N-O bond angle of near 130’. The results from the TOF measurement are consistent with a parallel dissociation transition (A’ A’) through the BlA(2lA’) state. The large amount of rotational excitation in the N2product (Figures 2 and
-
1246 The Journal of Physical Chemistry, Vol. 97, No. 28, 1993 4) and the strong temperature dependence on the photodissociation probability (Figure 5) are also consistent with the dissociation
occurring via a vibronically allowed transition through a bent excited state. B. Energetics. A reasonable estimate of the translational energy of the N2 and O(lD) products can be made from evaluating eq 1. Since a rotationally cold (Trot< 30 K) molecular beam was used, all the energy in the photofragments results from the dissociation photons. At 203.17 nm, thepeakoftheN2rotational distribution, the photon energy is 6.10eV. The dissociation energy of the N - 0 bond is Do = 1.677 eV, and the electronic energy of the fragments is EE = 1.967 eV.5 The vibrational energy of the products is near zero, which is not surprising since the nitrogennitrogen bond length does not differ significantly between N20, re = 1.128 A, and N2, re = 1.0976 A. The average rotational energyoftheN2productis ( E R )= 1.4f0.1 eV,andtherotational energy of the O(*D) product is zero. Using these values in eq 1 yields an average total translational energy of the photofragments of ( E T )= 1.1 f 0.1 eV. The average translational energy of the O(lD) fragment is [mNI/(mN2+ mo)]( E T )= 0.67 f 0.06 eV, and the average translational energy of the N2 fragment is [mO/(mN2+m o ) ] ( E ~=)0.38 f 0.04eV. Oftheenergyavailable to the photofragments, 43 f 4% is in translation, 0%is in vibration, and 57 f 4% is in rotation. The corresponding translational energy distributions are shown in Figure 4. These distributions were determined by using a Gaussian fit to the rotational distribution and repeating the calculations described above. These distributions clearly illustrate the dramatic difference between the translational energy of N2and O(lD) and the rotational energy of the N2 product. The values calculated for the total average translational energy of the O(lD) and N2 photofragments agree very well with the values determined by Felder et aL9 and Springsteen et a1.lO but differ somewhat from the values measured by Shafer et al.4 Felder et al. and Springsteen et al. each determined that 42% of the available energy is deposited into translation of the products for dissociation a t 193 nm, and Shafer et al. concluded that 96% of the available energy is deposited into translation for dissociation at 205.47 nm. In addition, Felder et al. and Springsteen et al. measured j3 = 0.48 f 0.02 and j3 = 0.50 f 0.05, respectively, while Shafer et al. measured j3 = 2. The rotational energies of the N2 products presented here confirm the measurements of the translational energy of the products by Felder et al. and Springsteen et al. Furthermore, the presence of 1.4 eV of rotational energy suggests that the excited state is bent and that the fragments do not recoil exclusively along the direction of the transition moment. This result is consistent with the j3 = 0.5 measured by Felder et al. and Springsteen et al. but not with the j3 = 2 measured by Shafer et al.
V. Conclusions We have measured the state-resolved photodissociation of N 2 0 a t 203-205 nm. The data are consistent with one-photon dissociation occurring via the bent 21A’(BlA) excited state. The N2 fragment is highly rotationally excited as a result of the excited-
I-
.
.
-*
nanism ana Kummei state geometry. The photodissociation probability increases substantially as the temperature is increased from 300 to 1500 K. The N 2 product has an average rotational energy of 1.4 f 0.1 eV and an average translational energy of 0.38 f 0.04 eV. Novibrationally excited N2 is observed. The O(lD) product has an average translational energy of 0.67 f 0.06 eV. The rotational energy of the products accounts for 57 f 4% and the translational energy accounts for 43 f 4% of the energy available to the products. Acknowledgment. The authors greatly appreciate several instructive conversations with P. L. Houston and thank him for making hismanuscript (ref 10) available tous prior to publication. We would like to thank A. Leone of Lockheed for the use of pulsed Nd:YAG laser. We also thank D. Friedrich of Battelle P N L for the use of the variable attenuator and T. Grogan for the useofthe powermeter. This project wassupported by theNationa1 Science Foundation under Grant C H E 88-13805. T.F.H. gratefully acknowledges support from a National Science Foundation Graduate Fellowship. References and Notes (1) Preston, K. F.; Barr, R. F. J. Chem. Phys. 1971, 54, 3347. (2) Paraskevopoulos,G.; Cvetanovic, R. J. J. Am. Chem. Soc. 1969.91, 7572. (3) Zhu, Y.-F.; Gordon, R. J. J. Chem. Phys. 1990, 92, 2897. (4) Shafer, N.; Tonokura, K.; Matsumi, Y.; Tasaki, S.;Kawasaki, M. J. Chem. Phys. 1991, 95, 6218. (5) Okabe, H. Photochemistry of Small Molecules; Wiley-Intersciencc: New York, 1978. (6) Ashfold, M. N. R.; Macpherson, M. T.; Simons, J. P. In Topics in Current Chemistry; Boschke, F. L., Ed.; Springer-Verlag: Berlin, 1979; Vol. 86, p 1. (7) Bersohn, R. J. Phys. Chem. 1984,88, 5145. ( 8 ) Schinke, R. Annu. Rev. Phys. Chem. 1988,39, 39. (9) Felder, P.; Haas, B.-M.; Huber, J. R. Chem. Phys. Lett. 1991, 186, 177. (10) Springsteen,L. L.;Satyapal, S.; Matsumi, Y.;Dobeck, L. M.; Houston,
P. L. J. Phys. Chem., preceding paper in this issue. (11) Zare, R. N. Mol. Photochem. 1972, 4, 1. (12) Sivakumar, N.; Burak, I.; Cheung, W.-Y.; Houston, P. L.; Hepbum, J. W. J. Phys. Chem. 1985,89, 3609. (13) Sivakumar, N.; Hall, G. E.;Houston, P. L.; Hepbum, J. W.; Burak, I. J. Chem. Phys. 1988,88, 3692. (14) Miller, R. L.; Kable, S. H.; Houston, P. L.; Burak, I. J. Chem. Phys. 1992, 96, 332. (15) Stowlow, A.; Lee,Y. T. J. Chem. Phys. 1993, 98, 2066. (16) Hopper, D. G. J. Chem. Phys. 1984,80,4290. (17) Monohan, K. M.; Walker, W. C. J . Chem. Phys. 1975,63, 1676. (18) Selwyn, G.; Podolske, J.; Johnston, H. S. Geophys. Res. Lett. 1977, 4, 427. (19) Selwyn, G.; Johnston, H. S. J. Chem. Phys. 1981, 80, 4290. (20) Hanisco, T. F.; Kummel, A. C. J. Phys. Chem. 1991, 95, 8565. (21) Hanisco, T. F.; Yan, C.; Kummel, A. C. J. Chem. Phys. 1992,97, 1484. (22) Kohn, D. W.; Clauberg, H.; Chen, P. Rev. Sci. Instrum. 1992,63, 4033. (23) Lykke, K. R.; Kay, B. D. J. Chem. Phys. 1990, 92, 2614. (24) Lykke, K. R.; Kay, B. D. J. Chem. Phys. 1991, 95, 2552. (25) Pratt, S.T.; Dehmer, P. M.; Dehmer, J. L. Phys. Rev. A 1991.43, 4702. (26) Herzberg, G.Molecular Spectra and Molecular Structure IIh Van Nostrand: New York, 1967. (27) Penn, S. M.; Hayden, C. C.; Muyskens, K. J. C.; Crimm, F. F. J. Chem. Phys. 1988,89,2909. (28) Chu, J. J.; Marcus, P.; Dagdigian, P. J. J. Chem. Phys. 1990,93,257.
