J. Phys. Chem. 1993,97, 4891-4898
4891
NH/ND( alA, v”) Vibrational Distributions in the UV Photolyses of HNJDN3 and HNCO/DNCO B. Bohn and F. Stuhl’ Physikalische Chemie I, Ruhr- Universitcit Bochum, 0-4630 Bochum, Germany Received: December I , 1992; In Final Form: February 22, I993
NH/ND(a’A,v’? radicals were generated in the 248- and 193-nm photolyses of HN3 and DN3 and the 193-nm photolyses of H N C O and DNCO. The vibrational population distributions of the imidogen radicals were investigated in each case. The populations from the photolysis of DNCO and DN3 are reported for the first time. The distributions among the vibrational levels barely exhibit isotope effects. Those obtained for HN3 are at variance with most of the literature values. That for H N C O does not agree with a reported result. Evidence from previous investigations of the products of relaxation and electronic quenching of NH(a,v”) is offered in support of our results. The vibrational distributions are consistent with dissociation processes on steeply repulsive potential surfaces.
Introduction The nascent internal energy distribution of photolysis fragments is one of the useful indicators in the study of the dynamics of photodissociation processes. The method of choice for mapping these distributions often is laser induced fluorescence (LIF). One of thesimplemoleculesthe photodissociationofwhich has recently aroused great interest is hydrazoic acid, HN3.1-22 The fragmentation into NH(aIA) and N2(X’2i) is one of its major dissociation While the internal energy of N2(X) is more cumbersometodetermine,lONH(a)is quite easily detected by laser light in a convenient wavelength region, 320-430 nm.13326 During previous years various results have been published on the vibrational distribution of NH(a,v”) for photolysis wavelengths ranging from 308 to 193 nm.1J3J4.20J6-29 The summary of these data is displayed in Table I. It also includes the results of this work. All displayed data were obtained by LIF methods. It should be noted that LIF signals had to be compared for quite different laser wavelengths in most of these experiments,because various vibronic transitions had to be used to cover the relatively large range of vibrational excitation. During some of the experiments,dyes had to be exchanged, and, in others, frequency doubling was applied for one of the compared LIF lines. One immediately recognizes in Table I that one has learned with time that a substantial fraction of NH(a) is formed in vibrationallyexcited states. However, the agreement for a given wavelength is unsatisfactory, and a trend with photolysis wavelength is not apparent. Being aware of this situation, we have reinvestigated the NH(a,v’? vibrational distributions in the photolysis at 248 and 193 nm. We have further studied the corresponding distributions for HNCO at 193 nm and the deuterated speciesof both parent molecules. This reinvestigation became feasible, because of new quantitative spectroscopic data, i.e. transition probabilities, predissociation efficiencies, and fluorescence lifetimes.30 And, furthermore, this study became necessary in the course of a study of the quenching of NH(a) by O Z , ~wherein ’ the question aroses as to whether or not the quenching process is influenced by vibrational excitation of NH(ap”). Exact knowledge on the degree of vibrational excitation and possible relaxation processes in NH(a,u”) is definitely required. Although Table I shows that the present values are in agreement with someliteraturevalues at best within thecombined error limits, we will furnish evidence from more indirect experiments in support of our data of vibrational excitation.
Experimental Section MetastableNH(aI A,v”) radicals were generated by photolyzing slowly flowing mixtures of HN3 or HNCO in large excess of He 0022-3654/93/2097-4891$04.00/0
or sometimes Ar. The deuterated species were treated correspondingly. The detection method of NH/ND(a) was LIF. The apparatus used has been described previously in a study of properties of NH(clII) states.30 The photolysis was performed with Xph = 248 (KrF) and 193 nm (ArF) excimer laser light. The energies per pulse were about 0.5 (KrF)and 0.25 J(ArF) at the exit of the laser. Due to the divergence (stable resonator) and the distance between laser and photolysis cell (3 m), the fluence in the observed photolysis volume was about 0.1 and 0.05 J cm-2 at 248 and 193 nm, respectively. By using a part of the expanded beam, an uniform radical concentrationis expected to be formed. In some experimentsthe divergence was significantly increased by placing a lens into the beam without having any effect on the results. The dye laser probed the photolysis volume at right angles to the excimer laser beam. The P(2) and Q(2) lines of the NH/ ND(clII, v’= 0 or 1 al A, v” = 0, 1,2, or 3) transifions were pumped. In somecases when the P(2) line was blended by another line, P(3) was chosen. The dyes and the transition wavelengths used in this study are listed in Table I1 together with some additional relevant information. The beam profile of the dye laser was expanded by a lens. An aperture of 0.4 cm2limited the beam cross-section so that a laser profile as uniform as possible was obtained. The fluence of the dye laser was adjusted by the position of the lens and by various attenuators. After transversing the photolysis cell, which was equipped with windows in positions of Brewster angles, the beam was collimated by a lens and stopped on the surface of a pyroelectricdetector (Gentec, ED-100). The laser energy per pulse was registered by a storage oscilloscope (LeCroy, 9410) and averaged for the time period of the dye laser scan. We are aware that this detector arrangement measured total light and did not discriminate laser light against nonlaser light. For small laser intensities the signal of the detector was additionallyamplified by a factor of 100 A 1%(Gentec, EDX-1). The transmission of the combination exit window of the photolysis cell and the lens in front of the pyroelectric detector was determined to be >95% independent of wavelength. However, for frequency doubled light, the transmission of the above combination was only 87 f 3%. The difference is due to the Brewster positions of the entrance and exit windows, which stayed in fixed positions, while frequency doubling rotated the polarization plane by 90°. This transmission was again found to be independentof wavelength. The fluenceof the laser light ranged from 1 to 1000 pJ cm-2. The delay between dye and excimer laser was constant at 10 1 s . Both lasers operated at a repetition frequency of 10 Hz. 0 1993 American Chemical Society
Bohn and Stuhl
4892 The Journal of Physical Chemistry, Vol. 97, No. 19, 1993
TABLE I: Literature Values and the Results of This Work on the Relative Vibrational Population of NH(a,v”) in the Photolysis of HN3‘ photolysis wavelength/nm 266 248 248 193 248 266 248 308 193 248 193 a
of‘=
0
1 1 1 1 1 1 1 1 1 1 1
relative population of NH(a,v’? vibrational quanta v”= 1 u”= 2
““a
3
ref 26 27 1 1 28 13 14,22 29 20 this work this work
0.72 f 0.22b 0.27 f 0.08b 1.1 f 0.3 1.47
0.8 0.3 0.53
0.9 f 0.5 0.12
0.66 f 0.05 0.53 f 0.12 0.48 0.1 1
0.09 f 0.05 0.085 f 0.029 0.21 f 0.07
0.017 f 0.007 0.1 3 f 0.05
The population is normalized for u” = 0. No entry indicates population below detection limit. Error limits from ref 48.
TABLE Ik Laser Dyes, P Lines Used,Measured Line Widths, and Saturation Wtsr dye; wavelength range’/nm
pumped line
observed line width (wavelength/nm)
sulforhodamine 101 (SHG),b 308-333
P(2) 0-0 N D P(3) 0-0 N H P(2) 0-0 N D P(2) 1-1 N D P(3) 1-1 N H P(2) 0-1 N H P(3) 0-1 N H P(2) 0-1 N D P(3) 0-1 N D P(3) 1-2 N D P(2) 0-1 N H P(2) 1-2 N H P(2) 0-2 N D P(2) 0-2 N H P(3) 1-3 N D P(2) 1-3 N H
0.45 (324.5) 0.43c (326.3) 0.37 (324.5) 0.33 (332.4) 0.3W (338.1) 0.21 (363.6) 0.23? (364.1) 0.20 (351.4) 0.22 (351.7) 0.34 (360.0) 0.25 (363.6) 0.32 (376.1) 0.24 (381.9) 0.21 (408.7) 0.13 (390.9) 0.22 (422.0)
DCM (SHG);b 316335d p-terphenyl; 332-3 50 DMQ; 346-377
TMI; 355-395 PBBO, 386-420 stilbene 3; 412-443
no saturation below/(pJ cm-2) 5 5 5
15 15 90 9oC 80
8oC 80 100 90
1o w 3 w 250/ 800/
a Wavelength range given by the manufacturer. S H G = second harmonic generation. Asmall influence of the &doubling on the bandwidth cannot be excluded. Upper limit determined by doubling crystal. The value of the P(2) line was adopted. f No saturation was observed. g The Q-lines used in the experiments are not listed in this table because their saturation effects were not studied (see text).
The dye laser was used to generate tunable light in the wavelength region between XL = 320 and 425 nm, partly (for XL < 334 nm) with the help of a doubling crystal (Lambda Physik, FL30). To ascertain whether or not the laser light is spectrally pure, the background radiation (ASE) was investigated by measuring the intensity with the blocked cavity end mirror of the oscillator. According to the laser manufacturer this procedure results in a maximum value of nonlaser background radiation. During these experiments, the ASE was measured to be typically 1% or less for decreased pump laser energies. p-Terphenyl, however, showed significantly increasing ASE toward shorter wavelengths and was therefore not used below 334 nm. Instead frequency doubling was applied (see Table 11). The laser induced fluorescence was detected by a monochromator (Jobin Yvon, H20) and a photomultiplier (EMI, 9789 QB). Two fluorescence wavelength regions were used, 325 f 10 and 305 f 10 nm, for the emissions due to the NH/ND(clII,u’ = 0 and 1 alA,u”= 0)transitions, respectively. The sensitivity of the optical arrangement was assumed to be the same for both wavelengths. The photomultipier signal was fed to a gated integrator (EGLG, 4152) and then to a personal computer. Typical delays of t , = 30-50 ns were chosen before the gate was opened. The delay time was measured after each experiment. The gate width, tgate,was adjusted to the fluorescence lifetimes of the upper states. The different input sensitivities of the gated integrator were checked by means of square pulses to be consistent within 1%. HN3 and DN3 were produced as described previously.~OHNCO and DNCO were generated by replacing sodium azide by potassium cyanate. Pressures of the parent molecules were between 0.05and 2 Pa in 3 kPa He. In some experiments, helium was replaced by 3 kPa Ar.
-
ReSUltS It turned out during this work that previously published work has not always been written so comprehensively that the causes for the observed discrepancies are apparent. We have therefore decided to describe in this section, in more detail, how we have obtained our results from the raw data and which assumptions and simplifications we have used. (a) Selection of Pumped Lines. Under the assumption of thermal equilibrium,N$/N,,,,,(u”), the fraction of population of a rotational level NH/ND(a,u”,J’? can be calculated from statistical thermodynamics for a given temperature T and by knowing the rotational energies F u ~ ~The p ~ molecular . data were taken from Hack and MilP and Cheung et al.33 The temperature, T, was chosen to be 300 K. This value isjustified by the relatively high pressure of He or Ar (3 kPa) and the rather long delay of 10 ps between generation and detection of NH(a), which is sufficient for complete rotational relaxation. It should be noted for a given value of P’that the fraction is only weakly dependent on d’because of the small anharmonicity of the potential. Full excitation spectra were taken for the 0-0,0-1,0-2,l-1,1-2,and 1-3 bands of the NH and ND(c,u’-a,v”) transition at saturation free conditions. An example of one of these 12 spectra is shown in Figure 1. The rotational line intensities of the LIF spectra of the 0-0 bands of NH and ND were evaluated and gave temperatures of 300 f 30 K (error: 3a) in support of the above assumption. Vibrational relaxation by He, on the other hand, was found to be negligible during the 10-psdelay. The various NH(a,u”) vibrational levels were observed to have equally long lifetimes of several milliseconds in 3 kPa He,34which are most likely caused by diffusion and/or radiation. In practice in the presence of the
NH/ND(aIA,u") Vibrational Distributions
The Journal of Physical Chemistry, Vol. 97, No. 19, 1993 4893 3001
380
381 382 383 excitation wavelengthhm
Figure 1. Excitationspectrumof the 0-2 band of the ND(c-a) transition. DN3 pressure: 0.43 Pa, Xph =. 193 nm, dye and excimer laser fluences: 1 mJ and 0.05 J cm-2,respectively; additional experimental parameters in the text.
parent molecules, the lifetime is determined by quenching. Although the removal rate constants for the various vibrational levels were measured to be slightly different for N H and ND,34 the small pressures of hydrazoic acid (at most 0.5 Pa) inhibited significantly different decays during the short 10-ps period. The pressures of isocyanic acid were higher (p < 2 Pa), but the rate constants were smaller and constant for all investigatedvibrational levels.34 We hence are safe in assuming that the populations of the selected rotational levels are a measure for the original vibrational distribution at the delay of 10 ps. Population ratios for two neighboring vibrational levels NH(a,u" and u" 1) were determined by comparison of the LIF intensities of two appropriate lines originating in the two vibrational levels. Figure 2 is an example of two such lines, P(2) 0-0 and P(2) 0-1 of ND(c-a) at about 324.51 and 351.42 nm, respectively. Although each line was scanned typically three times, only two scans of each line are displayed to indicate the reproducibility. The given time scale corresponds to the running time during an experiment, which will be converted to a frequency scale as described later. The profiles of the lines (data points) are approximated in Figure 2 by Gaussian curves (full line) from which the line widths were calculated, which are listed in Table I1 together with additional experimental parameters. It should be noted that these widths are dominated by those of the laser lines, since the Doppler widths of the absorbing lines are relatively small (-0.1 cm-I). Moreover, the A-splitting is rather small for the P(2) lines of N H and ND (0.033 and 0.009 cm-I, respectively).35J6 The A-splittings for the P(3)- and Q(Z)-lines are 0.1 cm-1 for N H and 0.03 cm-1 for ND35.36 and might influence the measured line width in the case of NH. The pumped lines were chosen to always be of the same type; i.e. in one experiment the P(3) line intensities were compared and, in an additional experiment, the intensities of the Q(2) lines. The first entry in Table I11 is an example for the selection of just these lines in two different runs. Because of reasons, which are explained later, always small values of J" were chosen. (b) Evaluation of the LIF Intensities. Our raw data consist of the output signal of the gated integrator, IGI,registered on the time scale, t , for a selected, constant laser wavelength scan speed, S = dX/dt = -(c (dv/v*)/dt). Thedye laser wavelengths, XL, were scanned very slowly ( S = 2 X 1 W nm s-l). In the mode of exponential averaging, the gated integrator averaged the signals from 10 single laser pulses during the time interval At = 1 s. Each second the data were transferred to the computer to result in an iQdividua1data point such as those shown in Figure 1. The sum over those values forming a line is IF,which is taken here as the measure of the fluorescence intensity. As Figure 2 shows, the measured data can be well approximated by a smooth curve, IGI(XL(t)),for example, of Gaussian shape. The intensity of a line, I F ,can thus be represented by the area under a I G I ( X L ( t ) ) profile for a scan from time t l to t2. For the integrated LIF
+
I
384
0
I
P(P)(O-O)ND
,
60
,
.
.
,
120
i
i
l
180"440
. . J 500 5€4
tiKl9lS
Figure 2. Profiles of the P(2) excitation lines of the 0-0 and 0-1 bands of the ND(c-a) transition. The signal of the gated integrator, IC!, is plotted vs time of the experiment,which is converted later to a frequency scan (see text). Of each line, two scans are shown to demonstrate the reproducibility. The experimentalconditions were 0.074 Pa DN3; Xph = 248 nm; Usate= 50 mV; and dye laser fluences, 3.1 and 15.9 pJ cm-2,
respectively.
intensity, IF, we obtain
In this equation, At = 1 s, VL is the laser light frequency and vc~~u' the mean rovibronic transition frequency of the pumped line originating in NH(a,v",J") and leading to the excitation of NH(c,u',J'). The relationship IF = S-I was checked for S = (2-7) X 10-4 nm s-1. In this run, the excitation occurred on the P(3) line of the NH(O-1) band. A very well defined linear plot was obtained leading almost exactly through the origin. The strength of the output signal of the gated integrator is determined by the chosen input sensitivity, U,,,,; gate width, tgate= t b - fa; gate limits, f a and tb; and the lifetime, q, of the pumped state and given by
G(vL)
where is the (extrapolated) photomultiplier signal at time is proporzero which thereafter decays according to T f . tional to the emitted photon rate at time zero: @(vL> = (constant)[+] dNh
G(vL)
i=O
(vL) =
(constant)N,,(t=O, V ~ ) A "(3), ~ In this equation, the constant contains parameters such as detection geometry, transmission efficiency of the optical arrangement, transmission of the windows, and photomultiplier data. Furthermore, Nu, is the number of radicals in the excited state NH(c,u') and Autois the transition probability for the two u'-O bands used in emission. It should be mentioned that, because of rapid relaxation, rotational states next to J'will be populated. This relaxation process will, however, not limit the use of eqs 2 and 3 because Autoand 71 were previously found to depend only weakly on J' for levels populated at room temperat~re.)~ For the further development of a useful expression, we assume an optically thin medium and consequently a locally constant spectral energy density of the dye laser, p ~and , a number density, Ni::/V, of the absorbing species NH(a,u",J'?, which stays constant in time during the pumping process. In particular, no saturation is allowed. Further, a homogeneous dye laser profile
4894
The Journal of Physical Chemistry, Vol. 97,No. 19, I993
Bohn and Stuhl
193
248
193
2jujil 1 Measured in the presence of 3 kPa Ar, otherwise 3 kPa He.
is assumed, fa,to be sufficiently large to avoid interference by the dye laser pulse, which is about 15 ns long, and a dye laser light pulse centered symmetrically around timet = 0. The extrapolated population of the pumped state, is then given by Nul,
Here FL and SE are the cross-sections of the dye laser beam and the observed length of the beam, respectively;ud'd is the absorption cross-section. For the purpose of a convenient mathematical solution, the given integration limits were chosen here and also later for the derivation of eq 7. The energy measured by the dye laser light detector, EL,after correctionfor the transmission of the exit window of the photolysis cell, is given by (5) The area under an absorption line, which consistsof a A-doublet, is given by
SJ'"are the appropriate Hbnl-London factors; Ad!,,and B , ~ ,are ,~
the Einstein coefficients for emission and absorption,respectively; and gAf,= 2 is the A-doublet degeneracy. For ud&) we assume a Doppler broadened absorption line shape (Gaussian), and PL( v , v ~ , tis) assumed to be of Gaussian profile in time and frequency. Introducing these shapes into eq 4 and consideration of eqs 1-3 gives
(7) Here Fgate = (Tf/Ugatetgate)(exP(-ta/Tf) - exP(-lb/.rf)}. The laser energyELwas taken tobe independent of V L for the small frequency interval of the pumped line. It should be noted that IF is independent of the bandwidth of the dye laser light. For quantitative LIF determinations of relative populations, it is necessary that saturation of the pumped line be negligible. This aspect was investigated in detail as demonstrated in Figure 3, for example, for the P(2) line of the ND(O-0) and the P(3) line of the NH(1-1) bands. For increasing fluence one easily recognizes nonlinearity of the fluorescence intensity. From the data represented in this figure it was decided that saturation is negligible for fluences of 5 and 15 pJ cm-*, respectively. Such saturation limits were obtained for all P(2 or 3) lines and are listed in Table 11. The Q-lines were not evaluated in this respect, because their Hbnl-London factors are smaller,37and therefore it appeared to be safe to use the P-line limits for them. In all 12 excitation spectra, for which Figure 1 is an example, the P(2 or 3) and Q(2) line intensities were compared. Their ratio was found to be according to the HBnl-London fact0rs3~as expected independent of the vibrational excitation. An intensity comparison of lines of the same type but originating from different vibrational levels therefore appears to be justified. (c) Vibrational Distributions. The fluorescenceintensities,I,, were measured in the following way: First the common type of pumped line was selected (P(2or3)) for two different vibrational
NH/ND(alA,u”) Vibrational Distributions 0
10
20
The Journal of Physical Chemistry, Vol. 97, No. 19, 1993 4895 probabilities and lifetimes of the NH(c,u? levels have been properly characterized bef0re.3~Moreover no differencebetween runs using He and Ar as buffer gases is observed. This indicates that diffusional effects and vibrationalredistribution by collisions with the bath gas are probably minor and the inert gas serves mainly to relax translation and rotation of the radicals.
Discussion dye laser fluence I pJ cm2
Figure 3. Fluorescence signal, IF,of two pumped lines, P(2) of the 0-0 of ND(c-a) and P(3) 1-1 band of NH(c-a), as a function of fluence of the dye laser. The upper scale is for the P(2) line and the lower for the P(3) line. The straight dashed lines reproduce the values at low fluences. The signals for both lines are not comparable, because of different experimental conditions. Relevant experimental conditions are given in the text and Table 11.
NH(a,u”) levels. The LIF intensities were obtained by consecutively scanning three times each of the two compared lines. This procedure was repeated six times for the chosen transition. Then a second type of line (Q(2)) was selected, and the same 36 line intensity measurements were performed. Thereafter the 72 measurements were repeated with roughly half of the dye laser intensityas an additional safetycheckwith regard tononlinearities. The result of this large number of experiments gives one entry in the last column in Table I11 where the measured population ratios of two compared vibrational levels are displayed. For the evaluation, the expression for the fraction N ~ ~ ~ / N , , , , , (and U ” ) eq 7 were used. In the majority of the experiments, a common upper NH(c,u’) state was chosen. Hence only the values of EL,and vdtut vary; all other parameters in eq 7 stay constant with the occasional exception of V,,,,. In some additional experimentsand when NH(a,u“=3) is involved, u’ = 1 and 0 were excited, as can be seen from Table 111. Appropriate values of Autoand F,,,,had to be taken into account in these cases. The literaturevalues necessary for the evaluation, r f and A,,p, were taken from Bohn et al.)O In accordance with the low rotational quantum numbers excited in this work, calculated transition probabilities for rotationless states were used. The lifetime T f is determined by radiation, predissociation, and quenching. For NH(c,u’=l), the calculatedvalue Tf = 63 ns was adopted.30 Rate constants for the quenching of NH(c,u’) by HNCO/DNCO are not known. Therefore in these cases, always a common upper state was pumped, as evident from Table 111. All the single population ratios given in the last column of Table I11 were finally placed on a common relative scale which is normalized for u” = 0, as in Table I. The result is displayed in Table IV. The errors given in Table I11 for the ratios were estimated on the basis of eq 7. For the reproducibility of the position of the dye laser beam, an error of *IO% was introduced. Additionally the following relative reading errors were estimated: Atgate/fgale = 0.05 and AEL/EL= 0.1. As mentioned before, the gain of the amplifier, the change of the sensitivity range of the gated integrator, and the nonlaser radiation each affected the precision by less than 1%. The relative errors of the lifetimes and the resulting errors of F,,,,and IF are listed in Table V. Neglected were uncertainties in the calculated transition probabilities and statistical fluctuations of the concentrationof the NH/ND(a,u”) photolysis product. The latter were kept small during the ratio measurementsby frequently changing the transitions and by the large number of runs. For the determination of the population ratios, those errors were estimated, which are given in Table 111. The assumption was that all error sources are independent. It should be noted that the reproducibility of the results was significantly better. The population ratios of Table I11 demonstrate that the results do not depend on pumping a common upper level, which of course simplifies the evaluation. This fact suggests that the transition &tu/,
The chosen photolysis wavelengths (kph = 248 and 193 nm) are those available in this laboratory. While the bond energy HN-N2isverysmall (1O.6eV)l5),that OfHN-COissignificantly (-3.6 eV)38larger and NH(a) + CO cannot be formed at 248 nm. The excess energy in the 248-nm photolysis of hydrazoic acid is much larger than that in the 193-nmphotolysis of isocyanic acid in spite of the different photolysis wavelengths. This is clearly reflected in the amount of vibrational excitation of the NH/ ND(a,u”) fragments (Table IV). The vibrational distributions of the deuterated and nondeuterated species are found tobevery similar. Thelargest difference is exhibited in the 193-nm photolysis of HN3/DN3, where consistently the population of NH(a,u’30) is slightly above, but always within the error limits of, that for ND(a,u’?. Such a fragmentationpattern is at variancewith a statistical distribution of the excess energy, Eavi.No other study of photolysis fragments is known by us to report on such vibrational distributions of isotopes, and no comparison appears to be possible. For example, the HF/DF vibrational distributions from the reactions of F + H2/D2 which channel much of the availableenergy into vibration are different.39 The relative vibrational NH/ND(a,u”) populations are plotted on a logarithmicscale vs the vibrational energy in Figure 4. This representation shows that the distributions for kph = 193 nm can be approximated by temperatures while, for kph = 248 nm, vibrational temperatures can be given, if the data points for u” = 0 are neglected. Equal distributions in terms of vibrational quanta means unequal energy distributions for the isotopes. This can be clearly observed in Figure 4 and in Table IV, where the last two columns list the average vibrational energy, Evib, and the fraction of -available energy deposited in the vibrations of NH/ND(a,u”), Evib/Eavl.In the case of HN3 this fraction is the same for the two photolysis wavelengths. The fraction of vibrational energy, of course, is found to be lower for DN3, but not the same for both wavelengths. Previously in our laboratory, fractions of 0.057 and 0.019 were determined in the photolysis of HN3 at 248 and 193 nm, respectively.’ In this case, only the first two vibrational levels were observed and the determinations were possibly not very precise as discussed later. The relatively small vibrational fraction of the excess energy is consistent with a direct process channeling a large part into translation of the fragments. Previous work on the photodissociation of HN3 at Xph > 230 nm is in support of a direct p r o c e ~ s . l , ~In~ the ~ ’ ~case of kph = 193 nm, it is not apparent from the ab initio calculations,17whether or not the dissociation leading to NH(a) N2 occurs on th_e lowest, excited singlet HN3(AIA”) surface or on one of the BIA’ and CIA’’ surfaces correlating with NH(b) and NH(c), respectively. In these latter cases, the efficient formation of NH(a) would be due to n~nadiabaticprocesses.’~ From the CO and NH(a) product distributions in the 193-nm photolysis of HNCO, Spiglanin and co-w0rkers38*~~*~~ concluded that the dissociation is direct and occurs on a repulsive potential surface. Ab initio calculations of the first excited HNJ(AIA”) surface indicate that a slight potential gradient can act on the H-N3 bond during the initial breaking of the HN-N2 bond.42 A small barrier found upon the H-N3 stretch43 might disappear upon relaxing the equilibrium geometry.42 In fact the HN3 photodissociation into H + N3 (or H + N + N2) takes place with quantum yields of 0.15 f 0.02 and 0.24 f 0.05 at kph = 193 and 248 nm, respectively.18 Classically this force, while acting on the
+
Bohn and Stuhl
4896 The Journal of Physical Chemistry, Vol. 97, No. 19, 1993
TABLE rV: Relative Quantum Yield (Normalized for NH/ND(a,vtt=O)) and Average Vibrational Energy, &it, relative vibrational population in u” photolysis wavelength/nm
3
kiblev
EwblEavi
(1) Photolysis of HN3 0.085 f 0.029 0.53 f 0.12 0.21 f 0.07 0.48 f 0.1 1
0.017 f 0.007 0.13 0.05
0.17 0.26
0.065 0.064
(2) Photolysis of DN3 0.12 f 0.04 0.16 f 0.05
0.024 f 0.010 0.074 0.029
0.14 0.16
0.054 0.040
(3) Photolysis of HNCO 0.031 f 0.010
0.094
0.074
(4) Photolysis of DNCO 0.054 i 0.016
0.088
0.069
1
0
2
248 193
1 1
248 193
1 1
0.51
193
1
0.26 f 0.05
193
1
0.30 f 0.06
0.10 0.38 f 0.08
E,,, to be distributed to translation of the fragments and rotation of NH(a,u’30). For HN3, a comparison of the data in Table I reveals that the present values are in agreement only with the previous ratio given from this laboratory for k p h = 248 nm, thanks to the previous large error 1imits.l~~~ Moreover, the most recent determination at Xph = 193 nmZoresulted in a distribution close Gust within the combined error limits) to the present one. How far this distribution is influenced by relaxation cannot be decided from this beam study. Further agreement among the literaturevalues is missing. There appears to be a trend of increasing vibrational excitation P with decreasing excess energy in the literature except for the 308-nm photolysis, which produces NH(a,u”=O) excl~sively.~~ On the other hand, the present data show a decrease of vibrational I I excitation with decreasing excess energy, perhaps with the 0 2 4 6 8 1 0 1 2 vibrational mergyll03cm‘ exception of DN3 which gives almost equal distributions for Xph = 193 and 248 nm (Table IV). Differences between the values Figure 4. Semilogarithmic plots of relative vibrational populations as a of Table I can be caused by a number of reasons. For example, function of vibrational energy. The populations are normalized to give a total observed population of 1 for hydrazoic acid and 0.1 for isocyanic in the previous study from our the ratio of transition acid. Temperatures are represented by straight lines. (a) Distribution probabilities for the 0-1 and 0-0 bands by Lents49was used which for NH/ND(a,u’? from the 248-nm photolysis of HN,/DN,. The is similar to that used in the present and 0 t h e r I ~ work, 9 ~ ~ however, respective temperatures are Tvlb = 2350 f 150 and 2100 200 K for different from that used by Hack and Mill,14322but these u”> 0. (b) Distribution for NH/ND(a,u”Jfrom the 193-nm photolysis differences alone cannot explain the various results. Previously, of HN3/DN, and HNCO/DNCO. The respective vibrational temperatures are Tylb = 6000 f 800 and 3700 i 300 K for hydrazoic acid and the conversion from wavelength to frequency was not performed, 3100 1000 and 2700 f 800 K for isocyanic acid. and the total dye laser fluence was measured in front of the c e l l l ~ ~ ~ and not the fluence across the observed photolysis volume. In TABLE V Estimated Relative Errors in the Determination fact, the comparison of fluences turned out to be the most difficult of 71, F,.,, and IF task, particularly after changing a dye and, at the same time, the pumped upper state beam position. The ASE was not considered in the previous relative error NH(c,o’=O) NH(c,u’= 1) ND(c,u’=O) ND(c,u’=l) workof this laboratory. Furthermore, the fluorescenceintensities were determined from an extrapolation to time zero, and the A71/rf 0.044 0.16 0.04 0.06 Mg,tc/Fg,tc 0.083 0.21 0.08 1 0.11 pumped lines were representative for a rotational distribution NFIIF 0.13 0.23 0.13 0.15 being less well d e f i n e d l ~than ~ ~ the present one at Trot = 300 K . Whether similar reasons are valid for the discrepancies between H/D-N3 bond for about the same short time, can be envisaged theliterature data is not evident, because of incompletelyreported to create about the same momentum in the H or D atoms, experimental details. irrespectiveof their masses; i.e. the kinetic energy channeled into We believe that indirect evidence for the degree of vibrational the bond is inversely proportional to the masses. An alternative excitation can be obtained from the growth of the NH(a,u”=O) explanation of the same vibrational distribution might be due to and NH(X,u=O) populations in systems which efficiently relax different or more or less confined Franck-Condon regionssampled thevibrational NH(a,u’? excitation before quenchingtakes place. on the excited HNJDN3 s~rfaces.~2 These different regions Such a system is NH(a,u’q + N2. While the quenching rate could perhaps affect the branching ratios into the NH(a,u) N2 constant for NH(a,u”=O) is relatively small (k = 8 X 10-14 cm3 vs H N3 (or H H N2) channels upon deuteration.” s-1),28*29J1,5031 the constant for the removal of u”= 1 is more than Vibrationaldistributionsof the deuterated ND(a,u”) fragments three times larger.14*29vs2 Relaxation into NH(a,u”= 0) is easily have not been reported before except for the generation of ND(a,u recognized by the initially nonexponential decay c~rves.14.28,~8,53 = 0 and 1) in the 193-nm photolysis of ND3.45 A previous study Vibrational relaxation appears to be the dominant removal of u” of the 193-nm photolysis of HNCO found only N H ( ~ , u ” = O ) . ~ ~ > 0, because no NH(X,u>O) is formed in the presence of N2,29.52 On the basis of this result, Spiglanin and c o - w o r k e r ~ ~have ~ . ~ . ~ ~ and the appearance rate of NH(X,u=O) is almost the same as the investigated the photolysis of HNCO at 193 nm. They find 20% removal rate of NH(a,u”=0).52 Modeling the nonexponential of Eaviin CO rotation and 12% of Eavlin NH(a,u”=O) rotation. decays at Xph = 248 nm gave a precursor concentration, Previously, Fujimoto et al.47have reported that about 14%of E,,] NH(a,u’30), of about 80 f 20% of that of N H ( ~ , U ” = O ) . ~ ~ ischanneled into CO vibration. Taking the present valueof -7% Preliminary experiments in our laboratory resulted in about 61 f 10% close to this last value and consistent with the present in NH(a) vibration leaves a remaining fraction of about 50% of 1
*
*
+
+ +
+
NH/ND(alA,u”) Vibrational Distributions
The Journal of Physical Chemistry, Vol. 97, No. 19, 1993 4897
TABLE VI: Comparison of the Relative Vibrational Distributions of the NH(X,r) Products in the Quenching of NH(a,v”) by Xe and the Present Values on the NH(a,P) Vibrational Distributions relative vibrational population
photolysis wavelength/nm 308‘b 266‘ 24gb 193‘ 193b a
U=O
v=l
1 1 1 1 1
0.46 0.08 0.53& 0.12 0.54 & 0.08 0.48 & 0.11
us2
* 0.04 0.029 **0.06 * 0.07
0.11 0.085 0.17 0.21
v=3
** *
0.017 0.007 0.07 0.05 0.13 0.05
u=4
*
0.03 0.02
ref 29 52 this work 20 this work
NH(X,u) product distribution. NH(a,v”) product distribution.
distribution but much too small for the large vibrational sociation can be problematic. This has also been experienced populations previously reported.13.14 recently for the OH(X,u) fragments in the 157-nm photolysis of Xe is a very efficient quencher of NH(a) (k = 1 X H20.57 Up to now, two significantly different vibrational cm3 s-1).20,29*52 The photolysis at 308 nm generates NH(a) exclusively distributions have been reported for this important dissociation in u” = 0.29 The only product of the quenching by Xe is process.58.59 NH(X,u-0). Inaccordancewith the abovereasoning, the product After submission of this work we were informed that the ratio of NH(X,u=O) in the presence of N2 and Xe has been wavelength dependence of the NH(a) u = 0 to u = 1 and u = 2 determined to be 1.0.29,54In the 248-nm photolysis, however, population ratio has been measured by Hawley et al. in the range the yield of NH(X,u=O) has been found to be 1Ss4and 1.5 f 220-290 nm?O The results are in good agreementwith the present data for HN3 at 248 nm. 0.P9 times larger in the presence of N2 than in the presence of Xe. Moreover, NH(X,u = 1 and 2) was observed.29 If one Acknowledgment. Financial support by the Deutsche Forassumes that Xe relaxes neither NH(a,u”) nor NH(X,u) but schungsgemeinschaft and Fonds der Chemischen Industrie is quenches while conserving vibrational energy, this ratio again gratefully acknowledged. The authors thank V. Staemmler for represents the total NH(a,u”) population compared with that in thorough discussions concerning the potential surfaces of HN3. u”= 0 in agreement with the data given above for the relaxation They also thank H. H. Nelson and P. J. Dagdigian for useful into NH(a,u”=O). Support for the above assumption is given discussions and W. Hack for sending a preprint of his and Th. below. Mill’s recent work (ref 22). Compared with theNH(a,u”) Nzvibrational relaxation rate, the removal rate by Xe is much No indication of References and Notes relaxation by Xe has been observed in the NH(a,u”) decays.29 According to published detailed rate constantss2 the rate of (1) Rohrer, F.; Stuhl, F. J. Chem. Phys. 1988,88, 4788. quenching of NH(a,u”) by Xe occurs synchronously with the (2) Foy, B. R.; Casassa, M. P.; Stephenson, J. C.; King, D. S.J. Chem. Phys. 1988,89, 608. rate of appearance of the same levels of v of NH(X,u). Following (3) Stephenson, J. C.; Casassa. M.P.;King, D. S.J. Chem. Phys. 1988, this argumentation, Hack and R a t h m a ~ ~concluded n~~ that 89, 1378. consent8Sljton of vibrational quanta can be assumed during the (4) Alexander, M. H.; Werner, H.-J.; Dagd$ian, P. J. J. Chem. Phys. i w , 8 9 , 1388. quenching of Xe and also N2. Similarly, Patel-Misra and (5) Foy, B. R.; Casassa, M. P.; Stephenson, J. C.; King, D. S.J. Chem. Dagdigian20state the suggestion that electronic quenchingby Xe Phys. 1989, 90, 7037. could be a vibrationally adiabatic process. If this is the case, the (6) Gericke, K.-H.; Theinl, R.; Comes, F. J. Chem. Phys. Leu. 1989, 164, 605. NH(X,u) product distribution mirrors the original NH(a,u”) (7) Yarkony, D. R. J . Chem. Phys. 1990, 92, 320. distribution, as noted by Patel-Misra and Dagdigian in their recent (8) Foy, B. R.; Casassa, M. P.; Stephenson, J. C.; King, D. S.J . Chem. study.20 Whether or not this then represents theoriginal fragment Phys. 1990,92, 2782. (9) Gericke, K.-H.; Theinl, R.; Comes, F. J. J . Chem. Phys. 1990, 92, distribution depends on the presence of additional relaxing 6548. partners. (10) Chu, J.-J.; Marcus, P.; Dagdigian, P. J. J . Chem. Phys. 1990, 93, We have therefore collected those few data available on the 257. NH(X,u) product formation during the quenching by Xe. They (11) Alexander, M. H.; Werner, H.-J.; Hemmer, T.; Knowla, P. J. J. Chem. Phys. 1990, 93, 3307. are summarized in Table VI together with the populations (12) Chen, J.; Quinones, E.; Dagdigian, P. J. J . Chem. Phys. 1990, 93, measured in this work for NH(a,u”). This set of data appears 4033. to be much more consistent than that of Table I. The total (13) Nelson, H. H.; McDonald, J. R. J . Chem. Phys. 1990, 93, 8777. (14) Hack, W.; Mill, Th. J. Phys. Chem. 1991,95, 4712. NH(a,v’q population compared with that of NH(a,u”=O) hence (15) Casassa, M. P.;Foy, B. R.; Stephenson, 3. C.; King, D. S.J . Chem. wouldbe 1,1.57,1.63, and 1.81 (or 1.82) for Apt, = 308,266,248, Phys. 1991, 94, 250. and 193 nm, respectively. These numbers increase noticeably (16) Gericke, K.-H.; Haas, T.; Lock, M.; Theinl, R.; Comes, F. J. J. Phys. with excess energy. Asimilar trend has been r ~ n t l y c a l ~ u l a t e d ~ ~Chem. 1991, 95, 6104. (17) Meier, U.;Staemmler, V. J . Phys. Chem. 1991, 95, 6111. and measured56 for the production of OH(g,u) in the vacuum(18) Gericke, K.-H.; Lock, M.;Comcs, F. J. Chem. Phys.Lerr. 1991,186, UV photodissociation of HzO. However, this set of data for 427. (19) Gericke, K.-H.; Lock, M.; Fasold, R.; Comes, F. J. J. Chem. Phys. NH(a,u’g would remove the very efficient production of vibra1992, 96, 422. tional excitation in the 248- and 266-nm photolyses. The work (20) Patel-Misra, D.; Dagdigian, P. J. J. Chem. Phys. 1992, 97, 4871. by Hack and Mill14 resulted in an inversion for u” = 1 at = (211 Meier. U.: Staemmler. V. To be aublished. 248 nm. These authors22suggest, on the basis of their transition (i2j Hack,’W.; Mill, Th. To be published. (23) Paw, R.J.; Bair, E.J. Int. 1. Chem. Kinet. 1976, 8, 139. probabilities, that the v” = 1 population given by Nelson and (24) Konar, R. S.;Matsumoto, S.;deB. Darwent, B. Trans. Faraday Soc. McDonald13at Xph = 266 nm could also be clearly inverted. Given 1971.67, 1698. these discrepancies, it would be highly desirable to understand (25) Okabe,H. Phorochemisrry ofsmall molecules; Wiley: New York, 1978. whether or not the p_hotolysisprocess HN3 NH(a) + N2 occurs (26) Baronavski, A. P.; Miller, R. G.; McDonald, J. R. Chem. Phys. 1978, on the same HN3(A1A’/)surface in the whole photolysis range 30,‘119. Xph 1 193 nm. In the case of a single surface, the dynamics (27) Dekoven, B. M.; Baronavski, A. P. Chem. Phys. Lett. 1982,86,392. (28) Hack, W.; Wilms, A. J . Chem. Phys. 1989.93, 3540. explaining the very different published distributions in the 248(29) Hack, W.; Rathmann, K. J. Phys. Chem. 1992, 96, 47. 266-nm region would then be of great interest. (30) Bohn, B.; Stuhl, F.; Parlant, G.; Dagdigian, P. J.; Yarkony, D. R. J. We have learned that the LIF determination of an original Chem. Phys. 1992, 96, 5059. (31) Freitag, F.; Rohrer, F.; Stuhl, F. J. Phys. Chem. 1989, 93, 3170. distribution of vibrationally excited products from a photodis-
-
+
r----
-
Bohn and Stuhl
4898 The Journal of Physical Chemistry, Vol. 97, No. 19, 1993 (32) Hack, W.; Mill, Th. J. Mol. Spectrosc. 1990, 144, 358. (33) Cheung, W. Y.; Gelernt, B.; Carrington, T. Chem. Phys. Lett. 1979, 66, 287. (34) Bohn, B.; Stuhl, F. J . Phys. Chem., to be published. (35) Ubachs, W.; Meyer, G.; Ter Meulen, J. J.; Dynamus, A. J. Mol. Spectrosc. 1986, 115, 88. (36) Hanson, H.; Kopp, I.; Kronekvist, M.; Aslund, N. Ark. Fys. 1965, 30, 1. (37) Herzberg, G. Molecular Spectraand Molecular Structure I . Spectra of Diatomic Molecules; Van Nostrand: New York, 1950. (38) Spiglanin, T. A.; Perry, R. A.; Chandler, D. W. J.Phys. Chem. 1986, 90, 6184. (39) Neumark, D. M.; Wodtke, A. M.; Robinson, G. N.; Hayden, C. C.; Lee. Y . T. J . Chem. Phvs. 1985.82.3045. Neumark. D. M.: Wodtke. A. M.:
Robinson, G. N.; Hayden, C. C.; Shobatake, K.; Sparks, R. K.; Schafer, T: P. J. Chem. Phys. 1985,82, 3067. T. A.; Perry, R. A.; Chandler, D. W. J . Chem. Phys. 1987, (40) Spiglanin, . -
87, 1568. (41) Spiglanin, T. A.; Chandler, D. W . J . Chem. Phys. 1987, 87, 1577. (42) Staemmler, V. Private communication. (43) Meier, U. Dissertation, Ruhr-Universitit Bochum, 1988. (44) Dagdigian, P. J. Private communication, 1992. (45) Kenner, R. D.; Rohrer, F.; Stuhl, F. J . Chem. Phys. 1987,86,2036. (46) Drozdoski, W. S.; Baronavski, A. P.; McDonald, J. R. Chem. Phys. Lett. 1979, 64, 421.
(47) Fujimoto, G. T.; Umstead, M. E.; Lin, M. C. Chem. Phys. 1982,65, 197. (48) Rohrer, F. Dissertation, Ruhr-Universitit Bochum, 1987. (49) Lents, J. M. J. Quant. Spectrosc. Radiat. Transfer 1973, 13. 297. (50) Bower, R. D.; Jacoby, M. T.; Blauer, J. A. J . Chem. Phys. 1987,86, 1954. (51) Nelson, H. H.; McDonald, J. R.; Alexander, M. H. J . Phys. Chem. 1990, 94, 3249. (52) Adams, J. S.; Pasternack, L. J . Phys. Chem. 1991, 95, 2975. (53) Reference 51 cited in ref 52. (54) Wilms, A. Dissertation, Universitet G6ttingen, 1987. (55) Engel, V.; Schinke, R.; Staemmler, V. J . Chem. Phys. 1988,88,129. (56) Mikulecky, K.; Gericke, K.-H.; Comes, F.J. Ber. Bunsen-Ges. Phys. Chem. 1991,85, 927. (57) Engel, V.; Staemmler, V.; Vander Wal, R. L.; Crim, F. F.; Sension, R. J.; Hudson, B.; Andresen, P.; Hennig, S.; Weide, K.; Schinke, R. J. Phys. Chem. 1992.96, 3201. (58) Andresen, P.; Ondrey, G. S.; Titze, B.; Rothe, E. W. J . Chem. Phys. 1984,80, 2548. (59) Mikulecky, K.;Gericke, K.-H.;Comes, F. J. Chem. Phys.Lett. 1991, 182, 290. (60) Hawley, M.; Baronavski, A. P.; Nelson, H. H. J . Chem. Phys., to be
published.
