6928
J. Phys. Chem. 1991, 95,6928-6932
Multiphoton Dissociation Dynamics of Dimethyl Selenide Joseph J. BelBruno,* Jennifer Spacek, and Elizabeth Christophy Department of Chemistry, Dartmouth College, Hanover, New Hampshire 03755 (Received: December 17, 1990; In Final Form: April 29, 1991) Dimethyl selenide photofragmentation by means of two-photon excitation through the structured, but dissociative, B'A, state hsa been studied via multiphoton-ionization time-of-flight mass spectrometry, UV spectrophotometry,and 2 + 1 REMPI. Atomic resonances indicating the formation of Se atoms were observed, and the formation of these atoms appeared to result from competitive dissociation channels. One of these channels involved the neutral fragmentation of the parent molecule followed by ionization of the fragments, while the second was characterized by direct ionization of the dimethyl selenide with subsequent fragmentation. Estimates of the rate constants for these processes were obtained from model calculations. Introduction
Metal alkyls have been the targets of increased research interest as possible precursors for vapor-phase deposition of metal atoms and alloys.' In particular, many of these compounds have potential uses as precursors for dopants in semiconductors, for building microstructures on silicon and other surfaces, and for growing binary semiconductors. The exact mechanism of the film production appears to be metal or alloy dependent. In some instances, gas-phase decomposition occurs and the metal atoms are subsequently deposited on the surface.2 Other films have been reported to grow as a result of surface heating or bandgap effects.' In any event, it is necessary to characterize the mechanism of the homogeneous gas-phase photochemistry of these materials in order to improve the systematic application of vapor-phase deposition to ml world problems. Gas-phase products, and especially organic materials, may deposit as impurities in the CVD film and change the characteristics of the desired material. The analogous Te molecule has been ~ t u d i e d , ' ~as * %have ~ the group IIB dialkylmetal~.~'It is of some interest to compare the Se results with both of these sets of data. In general, it is both practical and feasible to treat a dialkylmetal molecule, R-M-R, as a triatomic (with R = H) for purposes of theoretical correlations with any experimental data, even though umbrella vibrational modes of the methyl group may be observed under certain condition~.~ For group IIB molecules, this model triatomic is linear, but both theoretical and experimental considerations indicate that the selenium compound is bent with a C-Se-C bond angle of -96O. In the most simple version of this model we assume that each hydrogen (methyl group) contributes an s electron to a u MO and that the second electron in these orbitals arises from a metal atom orbital with very strong p character. The selenium atom also possesses two nonbonding p electrons. Several of the metal alkyls, for example Zn(CH3)2,Al(CH3)3, and Hg(CH3)2,have been well characterizeds-*in terms of both UV-vis spectroscopy and photodissociation dynamics. However, Se(CH&, DMS, is not well characterized. Although DMS has been employed in vapor-phase hemi is try,'^**^ its application as (1) (a) Didenkulova, 1.1.; Dyagilwa, L. M.; Tsyganova, E. I.; Alexsandrov, Y. A. Zh. Obsch. Khim. 1984. 54, 2288. (b) Stuke, M. Appl. fhys. Lerr. 1984.45, 1175. (c) Dyagileva, L. M.; Alexsandrov, Y.A.; Didenkulova, I. I.; Tsyganova. E. 1. Zh. Obsch. Khfm.1986.56, 1798. (d) Yablokov, V. A.; Dozorov, A. V.; Zorin, A. D.; Feshchenko. I. A.; Ronina, 0. V.; Karataev, E. N. Zh. Obsch. Khlm. 1986,56, 1571. (e) Brewer, P. D. Chem. fhys. Le??. 1987, 141, 301. (f) Ibuki, T.; Hiraya, A.; Shobatake, K.; Matsumi, Y.; Kawasaki, M. Chem. fhys. Le??.1989, 160, 152. (2) Rytz-Froidwaux, Y.; Salathe, R. P.; Gilgen, H. H.; Weber, H. P. Appl. fhys. A 1982, 27. 133. (3) (a) Fujita. S.; Tanabe, A,; Sakamoto, T.;Isemura, M.; Fujita, S.J . Crysr. Growrh. 1988, 93, 259. (b) Yoshikawa, A,; Okamoto, T.; Fujimoto, T.; Onoue, K.; Haseyama, K.; Yamaga, S.;Kasai, K. SPIE (Laser Process. Marcr.) 1989, 1190, 25. (4) Brewer, P. D.; Jenren, J. E.; Olson,G. L.; Tutt, L. W.; Zinck, J. Marer. Res. Soc. Symp. froc. 1988, 101, 327. (5) Mitchell, S. A,; Hackett. P. A. J . Chem. fhys. 1983, 79,4815. (6) Larciprete, R.; Borsella, E. Chem. fhys. Lei?. 1989, 147, 161. (7) Jackson, R. L.J . Chem. fhys. 1990, 92, 807. (8) bucrmann, Th.; Stuke, M. Appl. fhys. E 1989, 49, 145. (9) Johnson, W. E.; Schlie, L. A. Appl. fhys. Lett. 1982, 40, 796.
0022-3654191/2095-6928$02.50/0
a clean source of Se atoms has not been thoroughly examined and the mechanism (wavelength dependence, laser intensity dependence, pressure dependence, etc.) of the production of Se is not clear. This report describes our study of the dynamics of the photodissociation of dimethyl selenide. Experimental Section
The mass selective experiments described in this report were performed with a linear time-of-flight mass spectrometer of the Wiley-McLaren design.'*I2 The apparatus consisted of two independently pumped chambers, the source and drift tube regions, separated by an aperture. The source region consisted of a repelling plate and a pair of accelerating grids with the repelling plate and grid separated by 1.27 cm and the accelerating grids spaced 0.64 cm apart. The repelling plate was employed with an applied potential of 1750 V, the first accelerating plate was at 950 V, and the second accelerating plate was grounded. The sample was admitted to the source region via a molecular leak from a reservoir pressure of -50-200 Torr. The pressure in the source region, as measured by a cold cathode gauge, was always less than - 8 X lov5Torr, resulting in an estimated minimum mean free path in the source region of 0.25 m. While the total ion current increased with increasing pressure in the source region, the percent total ion current for each fragment remained constant until the source pressure was increased to approximately 5 X lo-' Torr. This is taken as evidence for the validity of the estimated mean free path. The drift tube was 91 cm in length, and the pressure in this region was maintained in the 10d-Torr range at all times. Ions were detected by means of a Galileo electron multiplier optimized for fast response, pulsed operation. The ion arrival time spectrum was sent directly to a transient recorder for signal averaging and manipulation. The time resolution of this device is 10 ns, and all reported traces are the average of lo00 laser pulses. The ion arrival time spectrra were converted to mass spectra by calibration using a mixture of chlorofluoro- and bromochlorofluorocarbonsand a computer analysis of the arrival times of the halogenated fragments. Laser multiphoton mass spectra (total charge vs amu) were obtained for nonresonant ionization at 355 nm via the third harmonic of a NdYAG laser (SpectraPhysics DCR-ll,6-ns pulse width). Wavelengths in the visible region were obtained by means of an XeCl excimer pumped dye laser (Lumonics T-861lEPD-300, 15-11s pulse width). Radiation at all of these wavelengths was focused into the source region by means of a 15 cm focal length lens. Pulse energies for the harmonics of the Nd:YAG ranged from 0.5 to 25 mJ per pulse. Experiments involving visible wavelengths were limited to pulse energies of the order of 1-3 mJ. In general, the resolution of the TOF bands is limited by the length of the field-free region and the operational parameters. A contributing factor to the width of the Se-wntaining TOF peaks
-
(10) (a) Wiley, W. C.; McLaren, J. H. Reu. Sei. Innrum. 1955,26,1150. (b) Wiley, W. C. Science 1956, 124, 817. (11) Siuzdak, G.; BelBruno, J. Appl. fhys. E 1990, 50, 221. (12) Siuzdak, G.; BelBruno, J. J . fhys. Chem. 1990, 94,4559.
0 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 18, 1991 6929
Dissociation Dynamics of Dimethyl Selenide
-' 6
Tr "-3
l.O--
I
I
I
Dimethylselenide
Se 4p4 3p2 + 4 3 5s" 3~
2
I OB--
0.6-Dimethyldelenide 3mlorr I 5 . 4 "
I 1
II
71,505
I 1
I
I
71,235
70,965
70,695
70,425
Three Photon Energy,cm-l Figure 2. REMPI spectrum (total current vs three-photon energy) over the energy range from 70400 to 71 770 cm-I. The spectrum has not been explicitly corrected for variations in the laser energy, but the dye laser output was approximately constant over this spectral range.
1 0
'Bent'
'Unear"
I
I
71,775
- v7
)imethylselenlde "6
"-11
o.o+
I I
51,030
I
I
45,430
I
I
39.830
em.'
Ffgure 1. One-photon UV spectrum of dimethyl selenide. The spectrum was obtained at Mom temperature with a sample pressure of 300 mTorr, an optical path length of 5.4 cm, and a spectral resolution of 0.2 nm.
is the fact that Se has five isotopes with natural abundance greater than 7%. Moreover, at some excitation wavelengths, metastable, unsaturated Se fragments are formed and contribute to the signal width. In all TOF spectra, other than those run using the third harmonic of the YAG,the slope in the early part of the signal is due to noise resulting from the spark gap of the excimer laser used to pump the dye laser. REMPI spectra were also recorded under static conditions using the dye laser previously described. The current vs absorbed energy signals were amplified, sent to a boxcar avenger, and subsequently stored in a microcomputer, which also controlled the scanning of the laser wavelength. Sample pressures of the order of 30 mTorr were employed in these studies. Single-photon UV-vis spectra were recorded with a resolution of 0.2 nm by means of a Perkin-Elmer Lambda 9 spectrophotometer. Sample conditions for this study included 300-mTorr pressure and a path length of 5.4 cm. Samples of dimethyl selenide were purchased from Strem Chemical Co.and were degassed and stored under vacuum in glass sample bulbs prior to use.
Ultraviolet Spectroscopy. The ultraviolet spectrum of DMS has not been previously reported and is presented in Figure 1 for a sample pressure of 300 mTorr. For energies less than 52 630 cm-I, two bands are observed. The low-energy feature, with a maximum at 42 550 cm-', is broad and only diffusively structured. The absorptivity in this region is relatively strong with c 3.1 X IO3 M-' cm-I at 42535 cm-'. However, the high-energy transition, which originates at 48 780 cm-I with what appears to be a hot band, is sharply structured and more intense. The data indicate that, for the strongest transition, at 49 310 cm-', t 1.2 X 104 M-I cm-'. The vibrational structure of this absorption band yields two average frequencies of 579 and 238 cm-'. REMPI Spectroscopy. Several portions of the REMPI spectrum of DMS are shown in Figures 2 and 3. The spectra shown represent total ion current vs transition energy. At energies less than -48 780 cm-', only sharp, atomiclike resonances were observed. The peaks in Figure 2 have been assigned (as indicated in the figure) to MPI of Se atoms from the ground electronic state
-
-
I
I
I
I
I
I
50,125
49.925
49,725
49,525
49,325
49,125
I
I
I
I
I
I
I
52,900
52,500
52,100
51,700
51,300
50,900
50,500
Two Photon Energy, cm.'
Figure 3. REMPI spectrum (total current vs two-photon energy) over the wavelength range from 50 380 to 53 330 cm-I. The spectrum has not been explicitly corrected for variations in the laser energy. The dye laser output was variable over this spectral range, so that the broad background represents a nonresonant signal following the trend in the laser energy variation.
of the atom. No atomic transitions other than those shown in this figure were observed. The sharp resonance at approximately 71 500 cm-' has been assigned as indicated in the Figure (4p35s" 3P).The attribution to Se is confirmed by the mass spactral data, but the exact spectral assignment is tentative and based upon the known energy levels of the atom and the appropriate spin and symmetry selection rules. The features of this region of the MPI spectrum were found to exhibit an P dependence on the laser intensity. The power indexes must be treated with caution. It has been shown that experimental conditions may play a major role in determining the apparent order of the process under study.') (13) Gandhi, S.R.;Bernstein,
R. B. Chcm. Phys. 1986, 105, 423.
6930 The Journal of Physical Chemistry, Vol. 95, No. I%, I991
BelBruno et al. TABLE I: Rehtive Yields of WOtoproducCs. at 10' W em-*
photolysis wavelength, nm
CH,
405.3
0.40 0.41
402.8
400.5 366.0 355
I
E
5
3
1 1 1 1 1 1 1 1 1 0
10
20
30
40
TOF. ps
[ I 5
I
'I 1 75
I 25
I
V
I 25
I
c
j
I 45
TOF, P
nFpc 4. REMPI-TOF mass spectra for 366.0-, 4005, and 402.8-nm excitation. nKlc wavelengths fall within the higher frequency absorption band shown in Figure 1. We have recorded such data and report the results here but apply these results in a purely qualitative manner; Le., we examine the index and appropriate energy level diagram to check for consistency. Mechanisticarguments are based on energetics and other experimental results. At higher energies (Figure 3). the REMPI spectra mimic the single-photon spectrum presented in Figure 1. All of the bands present under multiphoton excitation have been previously observed in the single-photon spectrum; however, the resolution of the REMPI spectra is considerably better than the spectrum in Figure 1. The low symmetry of the selenide ensures that almost all onephoton-allowed transitions will also be allowed for two-photon excitation. The vibrational frequencies derived from Figure 3 are consistent with those presented earlier. The signal in this region of the spectrum has an'I dependence on the laser intensity (with the caveats already noted). REMPI-Maaa speetnwehy. The photodissociation of DMS was studied via resonant excitation through the linear excited state and by means of nonresonant excitation at selected wavelengths both within this abeorption band and in other regions of the visible spectrum. The resulting TOF mass spectra for a selection of the wavelengths used in this study are shown in Figure 4. The TOF spectra show signals due to the multiple isotopes of selenium, including (CH3)$e+ ( m / e = 106,107,112, but primarily 78,80). In addition, there are strong hydrocarbon signals at m / e = 26, 27,29 (assigned to C2Hx+)and m / e = 12, 13, 14, 15 (assigned to CH,+). In general, the extent of fragmentation appears to increase with increasing laser frequency; that is, the relative ratio Se+:CH3Se+:(CH3)$e+ favors Se+ at shorter wavelengths. The CH,+ signal is intense at all pump wavelengths, and the signal assigned to C2Hxions is observed at all frequencies. Contrary to results for the group IIB dialkylmetal, the parent ion is observed at all excitation wavelengths. Relative yields of the photoproducts for a series of pump laser wavelengths are contained in Table I. The TOF spectrum at 400.5 nm (two-photon energy 49940 c"') resonant , pumping with C-Stc stretch excitation, indicata that the primary product in the photodissociation at this frequency is CH3Se. Figure 4 also contains the TOF spectrum for pumping with 402.8-nm radiation (two-photon energy 49650 cm-I). As is clear from Figure 3, this wavelength is not resonant with any vibrational band of the parent selenide and DMS has a very small absorption coefficient at this wavelength. The TOF spectrum shows some characteristic differences with respect to the mass spectrum produced by excitation in the region of the DMS stretching mode. There is almost no parent ion detected in this spectral region, but the relative quantities of CH3Se+and Se+ are similar to those obtained by excitation through the stretching frequency. As observed for 400.5-nm excitation, the major fragment in the TOF spectrum is CH3* and signals due to CzH, ions are readily observed. In general, the total ion yield (and by implication, the total photodiiation yield) is lower for excitation at 402.8 nm and for 400.5-nm irradiation.
Se 0.31
0.40 0.89
0.35 0.24 0.75
0.56
1.78
SeCH, 1 1 1 1 1
.oo .oo .oo .oo .oo
SC(CH,)~ 0.13
0.03 0.27 0.10 0.07
'Yields relative to monomethyl selenide. Since the peak shape depends on flight time, the yields should only be considered as an indication of the change in photoproduct composition, not as absolute measurements.
For shorter wavelength excitation, as shown in Figure 4, other changes in the TOF spectra may be observed. For example, at a pump wavelength of 366 nm (two-photon energy 54645 cm-I), very little parent ion is observed and nearly equivalent quantities of CH3Se+ and Se+ are produced, with the spectrum still dominated by the methyl ion peak. However, for 355-nm irradiation (two-photon energy 56 240 cm-') at an identical laser intensity, the majority of the photoproduced material is in the form of atomic selenium. The relative yields of the &-containing fragments were observed as a function of laser energy at 355 nm.
Discussion Comparison of the structure in the UV spectrum with earlier IR and Raman studiesl4 allows assignment of the observed vibrational frequencies to the v6 asymmetric stretch (579 cm-')and the v7 deformation (238 an-')of the C-Se-C bond. Vibrational ladders indicating excitation in the appropriate mode are superimposed on Figure 1. The ground state of DMS has C symmetry, as expected from a consideration of Walsh's rules.'? However, the presence or absence of vibrational structure is useful for the determination of geometries for the excited states. The presence of the long progressions in C - S t c bends is indicative of a change from bent (-96') to a more linear geometry in the high-energy transition. Similarly, one may postulate that the low-energy transition does not necessarily involve a significant change in geometry and that the broad character of this feature is simply a result of its dissociative nature. These assignments are consistent with both the observations of the UV spectrum of Hfie and a consideration of HzO and its Wabh orbitab.l5 The spectrum of H$e, assigned to a C , geometry, also exhibits a broad featureI5 as the lowest energy transition and a structured transition as the next observed excitation. The geometry of the latter has not been determined. However, H20, which has the same general spectral trends, has been analyzed. In terms of MO considerations, the first transition involves excitation of a lbl electron from the HOMO to the 4al LUMO, an n u* transition. The second, structured transition has been assigned as 3al 4al, leading to a change in geometry from bent to linear. This transition may be beat represented as an n, u* transition, where the u subscript indicates a nonbonding orbital with bonding character in the bent ground-state configuration. An analogous assignment may be made for H a and DMS, based upon the spectrum in Figure 1, and we have labeled the transitions as such, with the appropriate state symmetries, in the spectrum. Many of the energetic parameters for the DMS molecule and its photofragments are unknown. We have u t i l i the available information on the average bond energyI6and the ionization potential" of DMS, the energy levels1*of Se, and some available ionization data for related
-
- -
(14) Allkins, J. R.; Hendra, P. J. Spectrochfm. Acta 1966, 22, 2075. (IS) Herzberg, G. Mdecular Spectra and Molecular Structun; Van Nostrand Reinhold New York, 1966; Vol. 111. (16) Batt, L. In The Chemistry of Organic Selenium and Tellurfum Compounds;Patai, S., Rappoport, Z., Me.;John Wiley and Sons: London, 1986;Vol. 1, p 157. ( 17) Sturgeon, G. D.;Grosc, M.L. In The Chemistry of Ur@c Selenium and Tellurium Compounds;Patai, S., Rappoport, Z., Us.; John Wilcy and Sons: London, 1986;Vol. I, p 245. (1 8) M m ,C. E. Alomlc E n r h i s , Vol. II; Natl. Stand. Ref Data Natl. Bur. Sfand.) 1 8 NSRDS-NBS 35. Ser. (US.,
The Journal of Physical Chemistry, Vol. 95, No. 18, 1991 6931
Dissociation Dynamics of Dimethyl Selenide
T
3201 360
Se+ CH
280
'11
SeCH
,
40
91 PI
0
WCH3)*
Figure 5. Summary of the energetics of the SC(CH,)~system. The energy of up to t b n e 400- or 360-nm photons is a h indicated. See text for details of the various energy levels.
molecules19to construct the schematic summary in Figure 5. Note that since UV spectrum exhibits little absorption below -38 460 cm-l, irradiation of a sample with visible light may only lead to photodissociation via a two-photon process. The electronic transitions described above are symmetry allowedmfor both oneand twephoton excitation. The vibrational assignments labeled in Figure 3 correspond to those found in the one-photon spectrum shown in Figure 1. For the one-photon wavelengths used in this study, 355 nm < A < 435 nm, absorption into the "linear" state of *A, symmetry was the initial step in the photofragmentation process. The analysis of the spectrum indicates that absorption in this spectral region (or in the region of the 'Bl state) will result in a (CH3)& molecule with significantly weakened bonds because the excited electron is being promoted from a nonbonding to an antibonding MO. The REMPI spectra provide an indication of the neutral products arising from the MPD of the (CH3)zSe parent molecule. Figure 2 contains peaks that may be assigned to atomic Se transitions originating in the MJstates of the ground electronic state. No transitions from higher electronic states of the atom were observed in any region of the spectrum from 46 5 10 to 56 340 cm-I. The implications are that Se is a neutral dissociation product, hence its appearance in the REMPI spectrum, and that only ground-state atoms are produced in the dissociation. The REMPI spectra at shorter wavelengths contain resonances readily attributed to (CH3)#e. However, the TOF spectra indicate that the fraction of parent ion is quite small. Given the experimental data, including that shown in Figures 4 and 5, the (CH3)$e ionization potentiall' of 8.4 eV and an average C-Se bond energyI6 of 2.60 eV, the conclusion is that the second excited state of the parent (CH3)zSe molecule is dissociative and, at least at this wavelength, there are two competitive dynamical processes: neutral dissociation and molecular ionization. The presence of the nonresonant background, which may be assigned to a molecular ionization, in the REMPI spectra is a corroboration of this conclusion. The highsrder laser energy dependence reported for all of the REMPI spectra is also consistent with this conclusion regarding the nature of the excited state. Qualitatively, these power indexes appear to be consistent with the energetics of the photosystem. The transitions assigned to Se+ and CH3+depend on P. Figure 5 shows that this is the minimum number of photons (19) Yadav, V. K.; Yadav, A.; Poirier, R. J . Mol. Strucr. 1989,186, 101. (20) DiGiuceppe, T. 0.;Hudgenr, J. W.; Lin, M.C.J. Phys. Chrm. 1982, 86, 36.
necessary to produce ground-state Se and CH, by MPD and detect their presence by MPI. Moreover,the spectra in the region around 50000 cm-l exhibit an'I photon dependence. Reference to the energy level diagram indicates that only three photons are necessary to ionize the parent molecule. However, production and ionization of the CH3Se fragment are energetically possible with four photons for wavelengths at or below 400 nm. The TOF fragment laser energy dependence studies indicate that MPD and MPI processes are readily saturated. For example, the linear laser intensity dependence exhibited by Se+ appears to reflect saturation of all but one of the steps in the chain of photochemical p" beginning with absorption by CH3Seand proceeding through MPI of ground-state selenium. Relative timesf-flight photofragment yields were presented in Table I. The widths of the TOF peaks are not constant across the mass scale. Therefore, one must examine these integrated ion signals with reference to both indexes of the data matrix. They must be interpreted as a reflection of an overall effect and not as representing absolute measurements. In this light, it is clear that, over the 50-nm range of the data, a significant change in the yield of the atomic Se product is observed. For -400-nm irradiation, whether or not resonant with a parent molecule transition, the major photoproduct is CH3Seand the Se mass peak is only 1/4 to 1/3 of its signal. However, for radiation in the 350-370-nm range, Se is a major product. At the lower end of the wavelength range, it becomes the predominant fragment. At all pump wavelengths, the relative magnitude of the parent TOF peak is small; however, there appears to be a significant difference between resonant (400.5 and 405 nm) and nonresonant excitation (402.8 nm). The parent mass signal is vanishingly small at the latter wavelength, presumably due to photodecomposition of the parent molecular ion produced by three-photon nonresonant ionization. To summarize before considering the mechanism of this MPD process, (1) there are indications for fragmentation proceeding through both ionic and neutral mechanisms, (2) the yield of atomic product increases with increasing pump laser frequency and pump laser pulse intensity, and (3) the resonant mechanism appears to be initiated by excitation of a nonbonding electron into a u* orbital. A possible reaction mechanism consistent with experimental observations is presented as reactions 1-6. From the TOF spectra,
-
(CH3)$3e* + 3hv
(CH3)&+
+ 2hv CH3Se* + CH, CH3Se* + hv CH3 + Se CH3Se* + 2hv CH3Se+ Se + 2hv Se* Se* + hv Se+ CH3 + 3 h ~ CH3+
(CH3)+3e
-
-+
(1) (2) (3a) (3b) (4) (5)
(6) three-photon ionization of the parent molecule followed by the absorption of additional photons by the parent ion with subsequent ionic decomposition is a possible mechanism at all wavelengths. At resonant wavelengths, however, the relevant mechanism shown in reactions 2-6 is observed to dominate the dynamics. If the laser intensity is sufficiently low, the rate of ionization will decrease and fragmentation will dominate. As the laser energy is increased to larger values, of the order of 5 mJ/pulse, the parent ion absorbs additional photons and is readily fragmented so that the molecular ion is lost from the TOF spectrum. We speculate that, in the data presented in Figure 4, the parent ion is due to reaction 1, while most of the remaining signals result from the second, dominant mechanism. This second pathway also accounts for the large CH3+ peak observed throughout the course of this research. By this reasoning, background signals in the REMPI spectra may be assigned to nonresonant ionization of the parent molecule. Similarly, the dependence of the Se yield on the pump laser frequency is readily reconciled with this mechanism. As the frequency is increased, the excess energy deposited into the monomethyl sel-
6932
J . Phys. Chem. 1991, 95, 6932-6936
of 12 ns and an intensity of cm-2 s-', the typical value used in these experiments. The values for the cross sections are determined by comparison of the model results with the experimental ratios presented in Table I. In order to obtain product ratios consistent with those observed, values of the order of uI = 10-48 cm2 are necessary. It is important to note cm4 s and u2 = that this ratio is not sensitive to the value of u1but is extremely sensitive to the values of u2 and I. Given that the magnitudes of the cross sections are quite reasonable, we assume that the dynamics may be adequately described by the reaction mechanism presented above. The resulting rate constants for reactions 2 and 3a are equivalent, k2 IO8 s-' k3s. It appears that we are operating in a regime near the saturation limit (kt I), and this has been confirmed by intensity-dependent TOF spectra. The small quantity of parent ion detected in these studies is assigned to coherent three-photon ionization of the ground-state molecule in competition with the MPD process. Unlike studies involving Ga(CH&, this direct ionization pathway offers minimal competition for the MPD of the parent molecule even at laser intensities that saturate the dissociative reaction.
enide in reaction 2 also increases. Although at all wavelengths studied the initial excitation energy is sufficient to cause loss of both methyl groups, the RRKM rate constant for this process with 400-nm radiation is small. However, the excess energy increases to 2 eV for 360-nm excitation, with a corresponding increase in the rate constant. The experimental data in Table I indicate that the rate of reaction 1 is considerably slower than that of reaction 2 and the rate of reaction 3a increases relative to that of reaction 3b as the pump laser wavelength decreases. If we consider only the steps that constitute the MPD mechanism, reactions 2 and 3a, it is feasible to use a rate equation apprroach to obtain estimates of the cross sections for the individual steps of the mechanism. This type of analysis has been successful in previous studies in our laboratory2' and others.= The relevant differential equations (R = CH3) are d(R2Se)/dt = U ~ P ( R ~ S ~ ) ~
(1)
d(RSe)/dt = U ~ P ( R ~-Su21(RSe) ~)~ d(Se)/dt = u21(RSe)
(111)
We have observed the visible laser MPD of (CH3&3e in the one-photon range from 355 to 410 nm. The observations are adequately explained by a mechanism that begins with twephoton excitation of this molecule into a dissociative %nearmexcited state which produces CH$e and CH3 fragments. The monomethyl selenide fragment undergoes absorption of an additional photon with subsequent production of Se and an additional methyl radical. The increase in production of Se with decreasing laser wavelength may be attributed to more resonant excitation of the monomethyl selenide intermediate into the dissociative excited state. The results indicate that the use of this alkyl selenide in film deposition or in the creation of 11-VI alloys is a feasible process and deserves further study.
(Se) - exp(-u21t) - (u2/ulr) exp(-u,Pt) + (u2/uli) - 1 -exp(-u,Pr) - exp(-u21t)
(IV) In these equations, uI and u2 are the two- and one-photon excitation cross sections, I is the laser intensity, and t is the laser pulse width. The laser pulse is assumed to be a square wave with a width ~
~
~
~
~
~
~
(21) BelBruno, J.; Greenfield,S.R.; Carl, R. T.; Hughes, R. P. J . Phys. Chem. 1980,92,2480. (22) Mitchell, S. A.; Hackett, P. A.; Rayner, D.M.; Humphries, M.R. J . Chcm. Phys. 1985,83, 5028.
-
Conclusions
If we use the assumptions outlined below, eq I has a simple solution and the remaining equations are readily solved after obtaining this solution. The quantity of interest, for correlation with the experimental results in Table I, is the ratio (Se)/(RSe). This is given by We)
-
-
~
Acknowledgment. J.J.B. acknowledges the partial financial support of the National Science Foundation through Grants CHE-8521664 and CHE-8852168. ~
_
_
_
_
_
Registry NO. Se(CH,),, 593-79-3.
TImtResolved Photodissoclatlon of Three tert-Butylbenzene Ions James D.Faulk and Robert C.Dunbar* Chemistry Department, Case Western Reserve University, Cleveland, Ohio 441 06 (Received: December 28, 1990; In Final Form: April 23, 1991) Time-resolved photodissociation (TRPD) in the ion cyclotron resonance ion trap was used to observe the slow dissociation of tri-tert-butylbenzene ion at four wavelengths between 615 and 532 nm. Dissociation rates of the order of 5 X IO3 to 3 X 10' s-I were found for thermalized ions. An excellent fit to the observed curves was obtained by convoluting an RRKM rate-energy curve with the thermal distribution of ion internal energies at 375 K. RRKM parameters for the best fit were Eo = 1.2f 0.1 eV and AS' = -7 f 7 cal K-'. Although this is the largest ion for which energy-resolved dissociation kinetic data have been obtained, there was no indication of a breakdown in the quasiequilibrium theory description of the dissociation. For tert-butylbenzene ion and di-tert-butylbenzene ions, dissociation at visible wavelengths was found to be fast on the TRPD time scale ( 5 x lo5
Introduction The fragmentation kinetics of large ions, where the number of internal degrw of freedom exceeds 100, are interesting in at least three contexts. First, characterization of large molecules by mass spectrometry often depends on breaking up the sample ions into smaller, structurally characteristic pieces. Particle impact and photodissociation are increasingly being used to induce such secondary Author to whom correspondence should be addressed.
fragmentations. Understanding the rates and pathways of these processes is vital, but our extensive knowledge about fragmentation kinetics of medium-sized ions does not necessarily carry over well to large systems. Second, the quasiequilibrium theory (QET) of unimolecular decomposition' has been widely and successfully applied to the ( I ) Forst, W. Theory of UnimolccularReactions;Academic Press: New York, 1973. Robinson, P. J.; Holbrook, K. A. Unimolccular Rcactlons; Wiley-Interscience: New York, 1972.
0022-3654/91/2095-6932$02.50/00 1991 American Chemical Society
