J. Phys. Chem. 1993,97, 9916-9923
9916
Internal Energy Distributions from Nitrogen Dioxide Fluorescence. 3. Photolysis of Jet-Cooled N2O4 Wade N. Sisk,Charles E. Miller, and Harold S. Johnston' Chemical Sciences Division, Lawrence Berkeley Laboratory, and Department of Chemistry, University of California at Berkeley, Berkeley, California 94720 Received: November 22, 1991; In Final Form: April 14, 19920
A supersonic jet of Nz04 is photolyzed at three wavelengths: 351, 248, and 193 nm. The resultant NO2 fluorescence is dispersed, the fluorescence spectrum is folded into a cumulative sum, and the internal energy distribution of almost nascent photolysis products is found by the method of article 1 of this series. The spread of these product internal energy distributions increases as the photolysis energy increases from 351 to 248 to 193 nm. The most probable internal energy increases between 351- and 248-nm photolysis, but at 193-nm photolysis it is about the same as, or somewhat lower than, that at 248 nm. This apparent anomaly is explained in terms of the electronic states of the products. The internal energy distribution derived from 351-nm data is examined by the method of prior distribution, and the photolysis products are found to be one NO2 molecule in the ground electronic state and the other in the ,BI electronic state with 3 or 4 quanta of bending vibration excitation. The internal energy distributions at 193 and 248 nm are combined with Kawasaki's time-of-flight translational energy distributions at 193 and 248 nm, and upon adding considerations of molecular orbitals the product states are assigned for photolysis at 193 and 248 nm. From these considerations, it is concluded that the most probable N204 photodissociation channels are as follows: N z O + ~ hv (A = 351 nm) NO,* (B ,B1, v2 = 3 4 ) NO2 (X ZA1),N204 hv (A = 193 nm) NO,* (,B2) NO,* (~Bz), and NzO4 hv (A = 248 nm) NO,* (,BI) + NO,* (*Bz).
-
+
+
-
-+
+
Introduction The prompt fluorescence of NO2 following UV photolysis of N204has been examined in a number of previous studies.'-' The N204 ground state is planar symmetric D2h with an exceptionally long N-Nbondlength (0.1754.178 nmM). Thecorrespondingly weak bond dissociation energy at T = 0 is 12.88 kcal/mol(4504 cm-1).*t9 When N204is photolyzed by UV radiation, one NO2 or both fragments are produced in the excited state, emitting visibleradiation with lifetimes from 60 to 100pse9 Figure 1gives an energy ladder that shows the electronic states of the NO2 for 15 photolysis product channels.lJO The photon energies at four wavelengths, 351, 308, 248, and 193 nm, are included. The ultraviolet absorption cross sections are large, as indicated in Figure 2, which shows N2O4 to have a continuous spectrum including one peak at about 345 nm, a strong peak at 186 nm, and a shoulder implying a band between about 305 and 240 nm." It appears that the three laser wavelengths 351,248, and 193 nm fall close to the middle of these three absorption bands; that is, each excites a different upper electronic state. In previous NzO4 photolysis studies, the emphasis was on fluorescence quantum yields and translational energy distributions.1*2The N02* fluorescence continuum was observed. The flow cell work of Inoue et a1.2 suggests that channel 2 of Figure 1 is responsible for the fluorescence, following excitation in the range 295-365 nm. Photolysis of a neat supersonic jet of N2O4 has been carried out by Kawasaki and co-workers1 with the subsequent determination of the TOF translational energy distribution. They conclude that channel 3 is the predominant channel for 193- and 248-nm photolysis. We generalize the product channels of Figure 1 as
+
-
+
N 2 0 4 hv NO2(a) NO,(@) (1) One goal of this study is to obtain evidence for assigning a and B when N204 is dissociated at 351, 248, and 193 nm. For the shorter photodissociation 'wavelengths used in this study, it is ~~
*Abstract published in Advance ACS Abstracts. September 1, 1993.
0022-3654/93/2097-9916S04.00/0
51 .O
1 52 N 0 + 2 0
_193 _ -nm- _ - -
A 39.9
-
2
~
2
2
~
,
+ +
~
~
+
~
~
~
+
~
E.
28.8
7
7
6 17.6
22
~ 2 2+ ~ 1
6.5
1 2 ~ , +'A, -4.6
-
N2°4
Figure 1. Energetic thresholds for various N204 dissociation channels in terms of NO2 product states. The term symbols are NO2 BornOppcnheimtr electronic Btatca.
possible to produce one or two fluorescing NO2 fragments or one fluorescing molecule and NO + 0.
Experi~nentalSection A. Photolysis-InducedFluoresceace(PIF).1 . Moleculur Jet Conditions. Figure 3 illustrates the experimental arrangement for the N204 photolysis-inducedfluoreacence (PIF) studies, which includes a pulsed molecular beam cell especially built for these fluorescence studies. The electronic system is standard. The output of an excimer laser (Questek 2210) operating on XeF Q 1993 American Chemical Society
Photolysis of Jet-Cooled Nz04 I
The Journal of Physical Chemistry, Vol. 97, No. 39, 1993 9917 I
I
distance of the shock front from the nozzle, as a function of the above parameters:lZ
I
X =0 . 6 7 d a P
’
0.1 185
I
I
1
1
I
225
265
305
345
385
Wavelength/(nm)
Figure 2. Absorption cross sections of N204 and N01, adapted from ref 11.
(351 nm), KrF (248.5 nm), or ArF (193.3 nm) gas mixtures crosses a supersonically cooled molecular beam 8-10 mm downstream from the nozzle. A pulsed valve (NRC BV-1OOV) introduces the sample gas mixture into the chamber at a rate of 10 Hz. Its pulsing sequence is synchronously coupled to the excimer laser trigger through delaying electronics. The excimer trigger delay is adjusted to optimize the NO1 fluorescence signal, creating the best temporal overlap of the molecular beam and the 204s duration excimer pulse. The background pressure of the chamber was typically 7 X l e 7Torr when idle and maintained at a steady-state pressure of 2.5 X lW Torr during sample injection. For these experiments, the gas sample behind the nozzle consisted of 45-50 Torr of an NO2/N,O4 equilibrium mixture and 300 Torr of He buffer gas. Assuming a temperature of 298 K prior to expansion, the 50Torr NO2/N2O4 mixture contains about 16 Torr of N204. This leads to an estimated number density of 1.3 X 1015molecules/ cm3along the expansion axis 8 mm downstream from the nozzle. Neat N2O4 provided adequate signal levels but was unusually corrosive and damaged the pulsed nozzle. Despite the 2.7 X 1WTorr background pressure under steadystate operation, the shock front created in the supersonicexpansion produces a virtually collision free environment in the molecular jet. When the pressure behind the nozzle, PO,is large relative to the steady-state pressure, P,and the nozzle orifice diameter, d, is large, then the gas behind the shock front behaves as a free jet expanding into a vacuum and suffers essentially no collisionsdue to the background gas molecules. Equation 2 expresses A, the
The shock front calculated for a 500-pm-diameter orifice and the experimental expansion conditions is 36 cm, placing the laser/ molecular jet interaction volume well within the collisionless, free expansion region. 2. Fluorescence Collection and Detection. A 7.6-cm f / l Suprasil lens located 18 cm from the laser/molecular jet interaction region collected the NO2* fluorescence. A 2 cm X 2 cm aperture masked the lens to maximize rejection of scattered laser light while still collecting all possible fluorescence. The collection lens imaged the interaction region onto the entrance slit of a 1/3-m monochromator (McPherson Model 218). The light was dispersed with a 1200 lines/” grating blazed at 500 nm. An f/2 reduction lens imaged the exit slit onto the photocathodeofthePMT (eitherRCA4832GaAsorHamamatsu R1477 multialkali). The entrance and exit slits were set at 500 pm. Suprasil slit lenses of f/4.65 and f/4.25 maximized fluorescence collection through the monochromator, and a sharp cutoff filter (Corning 0-52) placed in front of the PMT photocathode eliminated all spurious radiation below 380 nm. The monochromator used in the present experiments operated in the continuously scanning mode. The exact wavelength increment between averaged data is not entirely controllable. This causes problems in the assignment of the fluorescence data wavelength in the PIF analysis but was handled in the following manner. Data were recorded for monochromator scan rates of 10.0 nm/min, and the pulsed valve and laser operated at a continuous 10-Hz duty cycle. The wavelength assigned to a given datum was the position calculated from the monochromator scan rate for 10.0 or 100 laser shots. It was later decided to average only the last 50 laser shots into every point to avoid spectral overlap with the previous point. The result is a scan which has a yspacing” of 2.13 nm between data. A three-stage 120-gain amplifier (Avantech GPD460 /GPD461/ GPD462) boosted the PMT signal before it was processed by a boxcar integrator (Stanford Research Systems 250). The boxcar output was interpreted by a 12-bit D/A card (Data Translation DT2801-A) and stored in an IMB AT personal computer. Two samples of the N02* fluorescence signal were recorded for every laser shot: one delayed 400 ns and one delayed 1500 ns from the laser molecular jet interaction. Each gate was 100 ns wide. The photodissociation event timing and shot-toshot laser power fluctuations were determined from the response of a fast Si photodiode. The experimental N02* fluorescnece signal was corrected for shot-to-shotvariation in the laser power as well as the wavelengthdependentspectral sensitivity of the entire optical detection system.
II
4...........
Chamber
p z J
c
Ngum 3. Experimental arrangementfor N204 photolysis-inducedfluorescencein a supersonicjet and an expanded view of the molecular beam chamber.
Sisk et al.
9918 The Journal of Physical Chemistry, Vol. 97, No. 39, 1993
For each phototube used, the system's spectral response profile was determined by projecting the emission of a tungsten lamp from the laser/molecularjet interaction region through the optical train. The system response calibration curve was generated by comparing the measured tungsten lamp intensities against the "theoretical" standard tungsten emission profile as a function of temperature and wavelength.*3-14The resulting system response curve corrected the observed N02* fluorescence signal for the wavelength-dependent photon detection efficiency of the optical train. The NO2/N2O4 sample (98% Matheson) was subjected to repetitive freeze-pumpthaw cycles until all discolorations had disappeared from the frozen solid, leaving all the white N204 crystals. The sample was stored at 77 K when not in use. 3. Cluster Formation. Upon expansion and cooling of NO2 in forming a molecular beam, the desired product N2O4 is formed, and undesired higher order clusters (Nodn may be formed. At the low pressures of the beam, the rate of formation of N2O4 is proportional to [MI [NO#, and the rate of forming (NOz). is proporational to [MI [NO2]",where [MI is total gasconcentration. The N-N bond energy in N2O4 is 13 kcal mol-', much stronger than the van der Waals bonds of higher order clusters, which favors N2O4 formation relative to higher clusters. We carried out calculations and experiments to find conditions that would produce some N2O4 and negligible higher clusters. In one experiment, the total NO2 fluorescence was measured as we scanned through the dissociation threshold of NO2, and we observed both NO2 LIF and N2O4 PIF in the same scan. In the room temperature cell before jet formation, the partial pressures were 14 Torr of NO2 and 2.8 Torr of N204, and the NO2/N204 ratio was 5. After supersonic expansion, the NO2/NzO4 ratio (determined from the LIF/PIF intensities) was 1.7, whereas virtually all NO2 would be converted to N2O4 at equilibrium at the temperature of the beam. With such a small conversion of NO2 to N2O4 and with consideration of the effects of pressure and temperature, we concluded that clusters would not be important under these conditions. A special experiment was carried through the courtesy of Professor Y.T. Leeand with theassistanceof his graduate student Pamela Chu. We used one of Prof. Lee's beam machines that includes mass spectral analysis. We found strong evidence of high-order clusteringwhen using neat NO2/NzO4 at pre-expansion pressures above 2OOTorr. Direct evidenceof high-order clustering could not be found when using 150 Torr or less of neat N02/ N2O4 or when using less than 100 Torr of NO2/N204 and 200Torr helium buffer. The experiments described here use 45-50 Torr of N02/N204and about 300 Torr of helium. These conditions appear to be safe with respect to formation of high order clusters.
Data Manipulation Internal Energy Distribution of a Fluorescing Population of Nitrogen Dioxide. The experimental N02* fluorescence signal was corrected for shot-to-shot variation in the laser power and for the spectral sensitivityof the entire optical detection system. These corrected data are formed into a cumulative sum, normalized to unit area under the range of observations. Much of the complicated NO2 absorption and fluorescence spectra defies quantum-state assignments. Demtroeder and cow o r k e r ~have ~ ~ recently reported that the spectrum is chaotic and follows a Wigner-type distribution. They conclude that this is a manifestation of the nonadiabatic vibronic coupling which destroys the Born-Oppenheimer description of the molecular eigenstates. The coupling is thought to be so extensive that only energy and angular momentum remain good 'quantum numbers" of the system. Our data-analyzing method of cumulative sum spectroscopy16considers only the internal energy of the highly excited N02*, not including explicit consideration of angular
0.26
1.05 Fit Gamma (SUM) 351 nm
0.21 c
e
w
1
9
0.16
0.63
2 0.10
3UJ
a
E
0.21
0.05
'11
0.42
14 17 20 23 Energy11000 cm"
26
'11
14 17 20 23 Energy/l000 cm-'
26
Figure 4. NO2 fluorescence, including reproduciblevibronic structure, upon NzOI photolysis at 351 nm. Solid lines: panel a, dispersed fluoresccncespectrum;panel b, cumulativesum of this spectrum. Dashed lines: panel b, analytic function fit to the cumulative sum; panel a, derivative of the analytical function fit to the cumulative sum.
0.14
8
f
0.10
2
2
0.07 v)
0.21
0.03
'11
14
17
20
23
26
"11
Energy/lOOO cm"
17
14
20
23
26
Energy/lOOO cm"
Figure 5. Same as Figure 4, except for 248-nm photolysis.
Fit Gamma (Data)
Fit Gamma (SUM)
- data
-data
2 0.40 U
UJ
0.20
'11
0.21
14 17 20 23 Energy/lOOO cm"
26
11
17 20 23 Energy11000 cm"
14
26
Figure 6. Same as Figure 4, except for 193-nm photolysis.
momentum. In this scheme the cumulative sum of the data is least-squares fit to a semiempirical analytical function (eq 10 of ref 16, the first paper in this series in this issue). A plot of the cumulativesum against energy gives a line that smoothly increases; the fine structure of the cumulative sum is largely obscured by the width of the line. Differentiation of the cumulative data sum recovers the full complexity of the fluorescence spectrum; differentiationof the fitted function gives a smooth curve, referred to as 'data fit". In this study of N2O4, we use the Gaussian weighting function ((36) of ref 16) and the kernel of the gamma function ((38) of ref 16). The final product is the relative distribution function for internal energy of the ensemble of molecules that fluoresce ((33), ref 16). RHultS
A. Internal Energy Distributions Inferred from the Data. Corrected for shot-to-shot variations in the laser power and the spectral responsivity of our optical detection system, Figures 4-6 give examples of the data at 351, 248, and 193 nm. The components of these figures should be read in the following
Photolysis of Jet-Cooled N204
The Journal of Physical Chemistry, Vol. 97, No. 39, 1993 9919 TABLE I: Electronic Origins and Vibrational Frequencies (cm-l) for NOf state origin W w2 w3
0.88
'AI 'Bz 'BI 'Az cm .'I1 000
Figure 7. NO'* internal energy distributions based on Gaussian and gamma kernel weighting functions following photolysis at 351,248, and 193 nm.
order: the photolysis-induced fluorescence (solid line, panel a), cumulative sum of this PIF (solid line, panel b), fit of cumulative sum of PIF to semiempirical function (dashed line, panel b), and derivative of fitted function labeled as "fit" to the fluorescence data (dashed line, panel a). The calculated fluorescencethreshold for the 35 1-nm spectrum is 23 890 cm-l (Figure l), which explains the absence of signal in Figure 4 at higher observation energies. We observe no substantial fluorescencefrom the 351-nmN204 photodissociation until about 2 1 000cm-1. The lower energy portion of the spectrum is dominated by a series of overlapping vibronic progressions. The cumulative sum, which rises above zero only for energies below 20 000 cm-', is a smooth, well-behaved function, and the observed sum and the fitting function overlap within the width of the line over most of the range. The data fit, the derivative of the analytical function fitted to the cumulative sum, has maximum intensity near 13 000cm-l. Detailed information about each experiment, the fitting parameters, and properties of the internal distribution functions are given in ref 17. The 248-nm PIF spectrum, Figure 5 , extends from the NO2 predissociation limit to our lowest observation energy, in contrast to the 351-nm PIF spectrum. It also has discernible vibronic structure superimposed on the emission continuum, but the intensityof thisstructure relativeto thecontinuum hasdiminished. Figure 5 shows that the most probable fluorescence intensity of the 248-nm spectrum is about 15 000 cm-l, which is larger than that of the 35 1-nm spectrum. These features all indicate that the fluorescingNO2 ensemble created in the 248-nm N204 photolysis is more energetic than its 351-nm counterpart. The 193-nm PIF spectrum shown in Figure 6 differs markedly from the 351- and 248-nm spectra. There is no evidence of individual vibronic features in the 193-nm spectrum. The 193nm PIF spectrum has the broadest distribution of fluorescence intensity. The fluorescence maximum has shifted back to lower energies and is now observed near 14 500 cm-l. The 193- and 248-nm cumulative sums are virtually identical until the observation energy reaches 22 000 cm-l, where they diverge with the 193-nm cumulative sum increasing less rapidly. Figure 7 displays the NO2* P(Eh,) derived from the each PIF spectrumvia its cumulativesum.16 All duplicate runs were pooled to produce these profiles. The abrupt termination of the P(Eht) contours at 25 130 cm-1 for the internal energy distributions at 248 and 193 nm marks the threshold for predissociation of NO2 into NO + 0. The Gaussian and the gamma kernel functions give very nearly the same distribution functions. On the basis of the gamma kernel function,the most probable internal energies of the fluorescing populations are 19 000, 22 OOO, and 22 000 cm-1 for photolysis wavelengths of 351, 248, and 193 nm, respectively. The spread of the distributions increases with increasing photolysis energy; the truncation of the populations at 248 and 193nm due to production of NO + 0 prevents calculation of the spreads for these cases. B. Prior Mstribution Comparison at 351 MI. The prior distribution1*is the statistical model employed to calculate the internal energy distribution of N02* from NzO4 photolysis at 351 nm. The prior distribution of states of the products has the
0 97100 147486 16293"
1320 1461 1192 1360
750 739 960 798
1617 1225' 1277' 135Oe
Reference 20. Reference21. w3 from [(WIW&'AI + (w10703)~&]/ [ ~ ( w I w ~or ) ~'All. B I Unlessotherwisespecified,thevalues areobtained from ref 19. relative probability for the product state proportional to the statistical weight, the number of different ways the energy may be distributed among the internal modes of the product, and translation of the two fragments relative to center of mass. Unlike phase-space calculations in which there are two constraints, angular momentum and energyconservation,the prior distribution operates solely under the constraint of energy conservation. Our cumulative sum method gives a probability distribution function of internal energy, and the prior distribution model, which also considers only energy, appears to be an appropriate model. The goal is to estimate the prior distributions of the photolysis product NOz(a), subject to an assignment of photolysis product (NO2(@),as defined by eq 1. When one applies the prior distribution model to photodissociation, the electronic states of product and reactants must be specified. Table I lists the NO2 parameters employed.1e21 We assign NO@) as the nonradiating ground state, N02[2Al], and we take NO2(a) to be each of 2B2, 2B1, and 2A2. The probability was calculated only for NO2(a) for the rotationless levels (J = 0), but the rotational degeneracy due to the N02(@)[2A~] fragment was taken into account. By assuming J = 0 values for NOz(a), we neglect the effects of Coriolis and ro-vibrational couplings in the population weights. The statistical weight for the NO2(a) fragment containing vibrational energy Ev(a) is given by
where E = the available energy after dissociation = E p - ~ Eb,,&, Ev(a) = the vibrational energy in the a NO2 fragment, E,(@) = the vibrational energy in the @ NO2 fragment, gv(a) = the vibrational density of states for the a NO2 fragment, gv(,9) = the vibrational density of states for the @ NO2 fragment, and P [E-Ev(a)-Ev(@)-Ej(@)]O.5proportionalto the translational density of states. The left panel of Figure 8 shows the experimental internal energy distribution derived from 351-nm PIF (compare Figure 7) and the prior distributions calculated for each of NO2 Z B ~~,B I , and2A2excitedelectronicstates. Clearly the2Bzprior distribution disagrees with the PIF P ( E N ~ ) The . 2B1 and 2Az prior distributionsresemble P(ENoJ much more closely but still display significant discrepancies in overall contour and the energy of the distribution maximum. We expect that the PIF results reflect no contributions from 2Az NO2 components since there is no symmetry-allowed electric dipole transition moment that enables this state to fluoresce to the 2A1 ground state. We assume that the linear N02(2B1) is born with nascent bending vibrational quanta in 2B1 state. The prior distribution model was adjusted to account for this by allowing the ~ Belectronic I origin to assume a new value of ~ B I u2, where u2 is the vibrational quantum number of the bending mode within the 'B1 electronic state. A family of curves for u2 = 2-6 is compared to the PIF distribution in the right panel of Figure 8. There is good agreement between
+
Sisk et al.
9920 The Journal of Physical Chemistry, Vol. 97, No. 39, 1993
,
1.20
11,01-
0.96
.-s n
0.72
2 10
14
18 22 cm.’ /i000
28
30
16
18
20
22
24
an’’11 ow
F i m 8. Left panel: The N02* internal energy distribution from 351nm PIF compared to prior distribution results assuming ZB2,2B1, or 2 A ~ electronicstates. Thesepriordistributionsarecomputedfromeq 3. Right panel: TheN02* internalenergydistributionfrom351-nmPIFcompared to prior distributioncurves assumingNO2electronicstate 2 B and ~ bending vibrational quanta given by u2 = 2, 3, 4, 5 , or 6.
thePIFpopulation and the prior distributionifthenascent product has 3 or 4 quanta of bending vibration, from which we conclude that the products are N0z(2A1) + NOZ(~BI, uz = 3 4 ) . Translational EnergV DistributionComparisonfor Data Taken at 248 and 193 nm. The PIF results at 248 and 193 nm are not suited for prior distribution calculations, because of the large number of accessible product channels and the possibility of the three-body channel: NO2 NO 0. Other considerations are used in these discussions. Kawasaki and co-workers’ have photolyzed a molecular beam of N2O4 and monitored the TOF of the NO2 fragment to obtain a translational energy distribution. Care must be taken in comparing Kawasaki’s translational energy distribution to the translational energy distribution derived from our PIF results. Subtractingthe PIF internal energy from the total available energy does not yield the translational energy, because the translational energy stated by Kawasaki is the total translational energy correlated to the total internal energy of the two postphotolysis NO2 fragments. The PIF energyof the fluorescing NO2 fragments does not correlate the two resultant NO2 fragments. Note in the PIF model the internal energy distribution may not exceed 25 137 cm-l, the dissociation threshold of NO2. The internal energy distribution from Kawasaki’s TOF results is for the two correlated NO2 fragments, which may exceed 25 137 cm-1 1; the internal energy from PIF is not necessarily correlated for NO2 fragments from the same parent. Kawasaki and co-workers1 suggest that at 193 nm photolysis of N2O4 is not a statistical process. We do not use the prior distribution method in an absolute sense, but usesomecomponents of it to make a four-way comparison: 193-vs 248-nm photolysis, equalityor nonequalityof energy states of the two nitrogen dioxide molecules. We have attempted toobtain the translational energy for PIF results as follows. We designate the correlated NO2 (NO2 fragments from N204 parent) as a and 8. The operating hypothesis of this model is that both NO2 fragments (aand 8)
+
+
have the same energy distribution. This model described the
distribution of the fragments in a statistical sense and does not say that both fragments from a particular N2O4 have the same energy distribution. The probability of a particular translational energy is the sum of the number of ways N02(a) and NO2(@) may possess particular energies within the constraint of the conservation of energy.
P(EN&)is the probability that an NO2 fragment has internal energy ENQ obtained from the PIF populations. Figures 9 and
’-=sm a“
0.48
0.24
a
3
6
9
12
15
Energy/l 000 cm“ Figure9. Observed NO2 translationalenergy distributionfrom Kawasaki et al. (ref 1) as curve A is plotted against the translational energy derived from internal energy distribution(compare Figure 7) of productsof N204 photolysisat 248 nm using eq 4 on the assumption that both NO&) and Not@) have the same electronic excitationenergy. The disagreement between the calculated and observed translational energies rejects the postulate of equal internal energies in the two NO2 molecules produced by the photolysis of N204 at 248 nm.
“0
6
12
18
24
30
Energy11000 cm-’ Figure 10. Similar to Figure 9, except the photolysis wavelength is 193 nm. In this case, the fair agreement between the observed translational energies and the translational distributions derived from PIF internal energy distributions according to eq 4 confirms the postulate that the two NO2 molecules produced by the photolysis of N204 at 193 nm have the same electronic excitation energy.
10show the comparison between Kawasaki’s translational energy distribution and that derived from PIF at 248 and 193 nm. The differences between the results at 248 and 193 nm are so large that they give a qualitative answer, which the nonstatistical features’ at 193 nm would not be expected to change. The calculated and observed translational energy distributionsof NO2 followingthe 193-nmphotolysisofN~04(Fi~ure 10)areconsistent with the postulate that both NO2 fragments from N2O4 at 193 nm have the same energy distribution. On the other hand, the disagreement between the calculated and observed translational energiesfor NO2 produced by N204 photolysisat 248 nm (Figure 9) refutes the postulate that the internal energy distributions are the same for NO2(a) and N02(@).
Photolysis of Jet-Cooled NzO4
The Journal of Physical Chemistry, Vol. 97, No. 39, I993 9921
N,O,
351 nm, PIF
?
4900 cm"
t
N02(2B,)tN02-
1
a,
w
19600 cm .1
-'
N204
9 I
Figure 12. Comprehensive energy diagram for N204 photolysis at 351 nm, showing energy levels of various products and showing (right-hand side) the internal energy distribution derived from cumulative sum PIF obtained in this study (compare Figure 7).
TABLE Ik Time-of-Flight Energy Terms (cm-I) for Nf14 Photolysis As Studied by Kawasaki et n1.I and Some Other Energy Terms X,nm hv
E1
Do Figure 11. NzO4/NOz molecular orbital correlation diagram based on von Niessen et al. (ref 22)with one modification at the top of the ladder (see text).
(hv + EI - DO)
Edmax)
Etdpeak) Eint(min)'
EinthWb Et-(max)/E.,il,
Discussion A. N204 Electronic Structure. The coordinate convention chosen here for the molecular orbitalsof N204/N02 is that utilized by von Niessen,zz Alrichs,z3 Pulay,24 and Bausch1icher:zs
2-2, Y-X N2O4 is in the XZ plane with the Z axis parallel to the N-N bond, and NO2 is in the I 2 plane with the 2 axis equal to the C2axis. D2,-.C,:
The NzO4 ground-state electron configuration is (4b3U)z(4bzg)2( b,g)2( 1aU)2(6a8)z Figure 11 shows a molecular orbital correlation diagram for NO2 + NO2 N2O4. 1. 351-nm Excitation. As an energy ladder, 15 possible product channels of N2O4 photolysis are listed in Figure 1. If the thermal energy of N204 is considered, channels 1-6 of Figure 1 are the only channels energetically possible at 35 1-nm excitation. Inoue et alSzconcluded from fluorescence quantum yield measurements that excitation into the first absorption feature (295365 nm) creates one NO1 fragment electronically excited and one electronically "cold", which eliminates channels 6 and 5. Channels 1 and 4 are eliminated due to lack of electric dipole moment (zeroth order) to allow fluorescence. Of the remaining channels (2 and 3), channel 3 was conclusively chosen over channel 2 in the prior distribution comparison (Figure 8). The electronic transition compatible with assigning 2BI as the electronic state of NO2* may be determined from the NzO4/ NO2 correlation diagram, Figure 1 1, based on the molecule orbital ordering of von Niessen et al.z2 (This ordering is utilized rather than the more conventional ordering, which places the 6blu
-
Eavail
"'
% %
~t-(pCak)/&dq
hv - 4600 E' E'- E-(max) E'- 4200
Et,,(max)/E', a
7%
193.3 51733 890 4600 48023 26 164 3510 21859 44513 54.4 7.3 47133 20969
55.5
Eht(min) = E.".il - Em,(max).
248.5 40240 890 4600 36530 10867 4180 25663 32350 29.7 11.4 35640 24773 3 1440 30.49
351 28490
890 4600 24780
23890
- E-(peak).
&,(peak) =
molecular orbital as the LUMO,because these authors are the only group to report quantitative energies and the ordering of the virtual orbitals. As presented here, Figure 11 has one assignment different from that of von Niessen). The lowest energy NzO4 electronic transition is 6a8 2bzUu ?F*,and it possesses a Y/bZ, transitionmoment andcorrelates to the2BI 2Al products. The calculations of von Niessen et a1.Z2 predict this to be the HOMO-LUMO transition. We assign it to the photolysis of NzO4 at 351 nm:
- -
NzO,
+ hv (A = 351 nm)
-
NOz* (B'B,, uz = 3-4)
+
+ NO, (X2A1)
The 35 1-nm photolysis results are summarized in Figure 12. Our assignment predicts a perpendicular transition moment for the first N2O4 absorption band and implies an angular distribution of photofragments peaked near 90°. 2. 193-nm N20, Photodissociation. Time-of-flight (TOF) and other energy terms are listed in Table 11. By combining Kawasaki et al.1 TOF distribution and our PIF internal energy distributions according to eq 4, we found evidence that the two correlated NO2 fragments, a and /3, have the same electronic states whenNzO4is photolyzed at 193 nm (Figure 10). Although channels 1-14 (Figure 1) are all energetically available, only
9922
Sisk et al.
The Journal of Physical Chemistry, Vol. 97, No. 39, 1993
channels 1,5,9,and 11obey this condition. Of these four channels, only channels 5 and 9 are allowed to fluoresce by electric dipole coupling. The TOF results yield a value of 26 164 cm-l for the maximum translational energy, El,,ru(max), which accounts for only 55% of the available energy as listed in Table 11. The minimum internal energy, Eint(min),determined by subtracting E,,,(max) from EaVafl is 20 969 cm-1. The only channel that satisfies these criteria and has a threshold below that of the 193nm Eint(min)is channel 5 (2B2 + 2B2). The energy difference between Eht(min) and 2B2 + 2B2 formation (20969-1 9500cm-1) is 1469cm-’. Dividing this energy evenly between the NO2 fragments places 735 cm-l in each NO2. This energy coincides almost exactly with the v2 bending frequency in ZB2 NO2. Such a dynamic motion is expected in the formation of ZB2 NO2 fragments, sinq the bond O N 0 bond angles in groundstate NzO4 are about 135O (same as those of NO2 2A1) and the bond angle in N02(2B2) is a more constricted 102°.26 The molecular orbital correlation diagram in conjunction with the TOF angular distribution from Kawasaki may be used to assign the molecular orbitals which correspond to channel 5 (2B2+ 2B2). The large absorption cross section at 193 nm is indicative of an electric dipole allowed transition.ll From the anisotropy of the NOz photofragments,Kawasaki and co-workers’determined the The only transition to be Z polarized (bl, symmetry in &A). electronictransition of bl, symmetry involving thevalence orbitals is the transition 6a, 6b1, u u*. Mason2’ has ascribed the 186-nmtransition to .(a8) u(bl,). Several additional arguments support this assignment despite the fact that this transition correlates to 2A1 + 2Al products. It is the only u u* transition available among the low-energy electronic transitions possessing the proper transition symmetry. The calculations of von Niessen et a1.z2mark the 6a, 6b1, promotion as the second-lowestenergy-allowed transition (not as the HOMO-LUMO transition, as several groups propose6.2*), making this assignment consistent with the absorption spectrum and the 6a, 2b2, assignment made for the 35 1-nm dissociation above. There appears to be a discrepancy between the assignment based on theoretical N204 electronic structure (6a8- 6blu, u us,ZAI 2A1products) and the 2B2 2B2 product channel based on ~ ( E N QP(Eh,,,), ), and energetic considerations. It is possible that the potential energy surfaces (PES)correlating to (2A1 2A1),channel 1, and (2B2 + 2B2), channel 5, products intersect. These surfaces must cross somewhereif the 6a8 6b1, transition is excited at 193 nm. The 2A1and 2B2 NO2 PESs cross for C, configurations with bond angles near 108°.39*m With more uncertainty than for the product assignment with 35 1-nm photolysis,we assign as products with 193-nmphotolysis of N2O4
- -
-
-
-
-
+
-
+
-
+
N 2 0 4 hv (A = 193 nm)
-
NO2* (’BJ
+
+ NOz* (2Bz)
and an interpretative energy diagram is given as Figure 13. 3. 248-nmNzO,Photodissociation. Upon photolysisof N2O4 at 248 nm, the available energy is 35 640 cm-I (Table 11). According to Figure 1, there are 12 energeticallyallowed product channels. Combining Kawasaki’s translational energy distribution with our internal energy distribution from PIF by means of eq 4, we found (a) the Eidmin) value of 24 733 cm-l (Table 11) eliminates channels 9-12 from consideration and (b) the two photolysis products do not have the same electronicenergy, EN%(a)# ENO@)(compare Figure 9). No nitricoxide was detected by Kawasaki in the 248 TOF experiment, implying that the NO producing channels and channels with only one excited NO2 are not significant (channels 1-4, 7). Channel 5 is eliminated by condition (b) above. The remaining choices consist of channel 6 (2B1 + 2B2)or channel 8 (2Bz+ 2A2). According to Table I1 and Figure 1, the value for Eint(min)(24 733 cm-l) agrees almost exactly with the
N , 0 4 , 193nm, PIF
.-
t l
5
0
*
21
P
e
w
14
- NO,(~B,)+NO,
3
Figure 13. Same as Figure 12, except for Nfl4 photolysis at 193 nm.
threshold energy for channel 6 (ZB1+ 2B2,24 450 cm-I), leading us to believe that this is the dominant channel at 248-nm photolysis of N2O4. The N2O4 absorption cross sectionsll show peaks at about 345 nm and at about 185 nm, and there is a shoulder between these two peaks that probably corresponds to a weak peak near 260 nm (Figure 2). The photolysis at 248 nm is within this third band, and it represents a different electronicstate than thoseencountered at 351 and 193 nm. The onset of the first absorption band of N2O4 is about 400 nm17 or 25 OOO cm-* (compare Figure 2), which is an upper limit on the transition energy of 6a8 2bzu. The onset of the second absorption band (shoulder of Figure 2) of N204 is about 305 nm or 32 787 cm-l. The onset of the third absorption band of NzO4 is about 240 nm or 41 667 cm-’. This transition is interpreted as 6a8 6bi,, and it gives the strong absorption at 193 nm. von Niessen et a1.22calculated that 2b3, is 15 000 cm-1 above 2bu. With this assignment, the relative energy (cm-l) spacings of states is 6a8, 0; 2ba, 25 OOO, 2b3,, 40 OOO; 6blu, 41 667 cm-1. This ordering is used in Figure 11. The molecular orbital correlation diagram (Figure 11) along with the TOF and PIF information may be utilized to discuss which molecular orbitals correlate to channel 6 (2B1 + 2B2). Kawasaki’s TOF angular distribution of N2O4 at 248 nm reveals a Z-polarized (bl, in Da) parallel transition. There are four possible transitions from the 4b5/4b3, orbitals to the 2bU/2bg orbitals, which correlate to 2B1 2B2. The lowest energy transition in this group is 4b5 2bU.” This transition is electronicallyforbidden. When the vibrational modes, as discussed by Bibart et al.41 and Snyder et a1.,42 are investigatedfor the possibilityof vibronically allowed transitions, no vibrational modes were found of the proper symmetry. MO calculations3~ indicate the 4b3, energy lies about 2 eV below the 4b2, energy, which makes the 4 b u 2 b utransition and the 4b2, 2b3, transition energetically inaccessible to 248-nm photon energies. The 4ba 2b3, transition, while also electric dipoleforbidden, produces an interesting vibronic transition. This transition requires an A, vibrational mode to occur via a bl, transition dipole moment. There is only one N204 ground-state vibration with this symmetry,the u4 torsionalbending mode, which destroys the planarity of the N204molmle and takesit toward a staggered configuration (Dw). The u4 has a very low frequency (70 (Snyder3’) or 79 cm-1 (BibarP)), and the torsional barrier is
-
-
+
-+
-
-
-
Photolysis of Jet-Cooled NzO4
The Journal of Physical Chemistry, Vol. 97, NO.39, 1993 9923
N2o4,248nm, 40 N20J -
"t
N02+NO+0
28
nm profile follows from the different NO2 electronic states of the products. We conclude that the 193-nm photolysisproduces NO2 in the Z B+~ ZBz state, whereas the 248-nm photolysis produces NO2 in the 2B1+ 2Bz state. This leads to the 248-nm population having a larger most-probable internal energy value, since the energy of NO2 ZB1lies well above that of 2Bz. The 35 1-nm results have a much larger portion of the available energy residing in the internal modes of N0z. From our results in the PIF experiment, weinfer that theTOF distribution is peaked near zero and extends to a maximum translation energy release of about 7000 cm-1. The internal energy distribution of NO2 obtained from the PIF deconvolution clearly shows a broadening of the internal energy distribution as the photolysisenergy increases. This could be due to a statistical contribution such that an increase in the photolysis energy would increase the number of ways the energy can bedistributedbetween internalmodesof NO2, thus broadening the distribution.
PIF
-
T
22100 cm"
241
7t
I
N02+N02
N204
OL
-
Figure 14. Same as Figure 12,except for N204 photolysis at 248 nm.
800-1000 cm-l (Snyder and Bibart). NzO4 dissociation from a nonplanar geometry has some interestingdynamical implications. If the 248-nm transition (40 323 cm-1) corresponds to the 4bzg 2b3, transition, the exciting energy includes the following components: (a) 4bzr 6ag plus (b) 25 000 cm-I (6a, 2bzu threshold, Figure 2) plus (c) 2bzu 2bsg plus (d) energy above the threshold that is reached by 248-nm radiation. von Niessen et aLZzcalculated that the 2b3, orbital lies about 15 000 cm-* above the 2b2, orbital. Since 40 323 cm-1 = (a) (b) (c) (d) and since (b) + (c) = 40 000 cm-1, these energy values imply that (a) + (d) is about zero, which is contrary to the observed absorption spectrum and to the MO ordering of 6ag > 4bo. If we assume that von Niessen et al.33 overestimated the 2b2, 2b3, energy separation and if the 4bz, level is no more than a few thousand cm-l below 6ag,the 248-nm transition could be 4bzg 2b3,, which correlates to 2 B ~ ZB2 as photolysis products. Another possible assignment for the 248-nm excitationof NzO4 is the transition 6ag 2b3,. The upper limit energy of the transition 2b2, 2b3, is the difference between the energy thresholds at 305 and 400 nm (Figure 2), which is 7787 cm-l or 0.97 eV. We must assume that von Niessen et a1.22 overestimated the 2bzu 2b3, energy separation, in which case it could be that the weak absorption of NzO4 for 248-nm radiation is given by the transition 6a, 2b3,. This transition does not correlate to the products zB1 + ZBz. and as for the case with 193-nm photolysis, we must invoke curve crossing to adhere to this assignment. In this case, the relative energy (cm-1) spacings of states are 6ag, 0; 2bu,25 000,2bpg, 32 787; 6blU,41 667. With substantial uncertainty, relative to the case at 351 nm, we assign the following as the overall reaction for photolysis of NzO4 at 248 nm:
-
- -
-
+ + +
-
-
--
-
+
-
NzO, + hv (A = 248 nm)
-
NOz*('BJ
+ NO2* ('BJ
and the interpretative energy diagram is given as Figure 14. 4 . NzO4 Photodissociation Wavelength Comparison. A few points can be made in comparing the photodissociation at the three photolysis wavelengths. The reason why the 193-nm PIF population profile has a lower internal energy peak than the 248-
Acknowledgment. This project was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Chemical Sciences Division of the US.Department of Energy, under Contact DE-AC03-76SF00098. We thank Professor Y. T. Lee and Pamela Chu for letting us do some experiments in their laboratory to determine conditions under which high-order (NO2)" clusters are and are not formed. References and Notes (1) Kawasaki, M.; Kasatani, K.; Sato, H. Chemical Physics 1983,78,
65. (2) Inoue, G.; Nakata, Y.;Usui, Y.;Akimoto, H.; Okuda, M. J. Chem. Phys. 1979,70, 3689. (3) Auwera, J. V.; Herman, M. J. Photochem. 1987,38, 15. (4) Smith, D.W.; Hedberg, K. J. Chem. Phys. 1956,25, 1282. ( 5 ) McClelland, B. W.; Gundersen, G.; Hedberg, K. J. Chem. Phys. 19!%,56,4541. (6) Kvick, A.; McMullan, R. K.; Newton, M. D. J. Chem. Phys. 1982, 76,3754. (7) Hisatsune, I. C. J. Phys. Chem. 1961,65,2249. (8) Giauque, W. F.; Kemp, J. D. J . Chem. Phys. 1938,67,81. (9) Patten, K. O.,Jr.; Burley, J. D.; Johnston, H. S . J. Phys. Chem. 1990, 94,7960. (10) Baulch, D. L.;Cox, R. A.; Hampson, R. F., Jr.; Kerr, J. A.; Troe, J.; Wataon, R. T. J. Phys. Chem. Ref Dura 1980,9,467. (11) Bass, A.; Ledford, A. E., Jr.; Laufer, A. H. J. Res. Nurl. Bureau Srands. 1975,80,143. (12) Campargue, R.;Lebehot, A. Rarefled Gas Dyn. Proc. Inr. Symp. 1974,2,C11-1. (13) DeVos, J. C. Physica 1954,690. (14) Pon, R. M.; Hessler, J. P. Appl. Opp. 1984,23,975. (15)Demtroeder, W.; Duchowicz, R.;Gress, J.; Forth, H. J.; Kullmer, R.; Persch, G.; Schwarz, M. Physica Scripta. 1988, T23, 176. (16) Johnston, H. S.;Miller, C. E.; Oh,B. T.; Patten, K. O., Jr.; Sisk, W. N. J. Phys. Chem., article 1 of the series in this issue. (17) Si& W. N. Ph.D. Dissertation, University of California, Lawrence Berkeley Laboratory Report 291 12, 1990. (18) Zamir, E.; Levine, R.D. Chem. Phys. 1980,52,253. (19) Lafferty, W. L.; Sams, R. L. J. Mol. Specrrosc. 1977,66,478. (20) Weaver, A.; Metz, R. B.; Bradforth, S.E.; Neumark, D. M. J . Chem. Phys. 1989,90,2070. (21) Innes, K. K. J . Mol. Specrrosc. 1982,96,331. (22) von Nicssen, W.; Domcke, W.; Ccdarbaum, L. S.;Schirmer, J. J. Chem. Soc., Faraday Trans. II 1978,74, 1550. (23) Alrichs, R.;Keil, F.; J. Am. Chem. Soc. 1974,96,7615. (24)Pulay, P.; Hamilton, T. P. J . Chem. Phys. 1988, 88, 4926. (25) Bauschlicher, C. W., Jr.; Komornicki, A,; Roos, B. J . Am. Chem. Soc. 1983,105, 745. (26) Jackels, C. F.; Davidson, E. R.J. Chem. Phys. 1976,61,2908. (27) Mason, J. J. Chem. Soc., Dalton Trans. 1985,I , 19. (28) Howell, J. M.;Van Wazer, J. R.J . Am. Chcm. Soc. 1974,96,7902. (29) Persch, G.; Mehdizadeh, E.; Demtroeder, W.; Zimmermann, Th.; Koeppel, H.;Cederbaum, L. S.Ber. Bunsenges. Phys. Chcm. 1988,92,312. (30) Bibart, C. H.; Ewing, G. E.J. Chcm. Phys. 1974,61,1284. (31) Snyder, R. G.;Hisatsune, I. C. J . Mol. Specrrosc. 1957,1, 139.
