J. Phys. Chem. 1982, 86, 4096-4107
4096
FEATURE ARTICLE Nonilnear Photochemistry in Organic, Inorganlc, and Organometallic Systems A. Gedanken,? M. B. Robln,' and
N. A. Kuebler
Bell LaLuuatorles. Murray Hill, New Jersey 07974 (Received: February 9, 1982; In Final Form: June 28, 1982)
A molecule struck by an intense laser pulse can undergo two general types of nonlinear photochemistry: ionization followed by fragmentation (class A), or fragmentationfollowed by ionization (class B). This classification scheme is applied toward an understanding of the MPI spectra and photochemistries of various class A systems (acetaldehyde, dimethylmercury, norbornadiene, and tetramethylsilane) and class B systems (chromium hexacarbonyl,the mercuric halides, tetramethyltin,the titanium halides, and silicon tetrachloride). Additionally, we discuss the MPI of stannic chloride, which shows class A and class B behavior simultaneously, and of methyl iodide, which is class A in one spectral region and class B in another.
In roduc ion UV/visible laser multiphoton ionization (MPI) of polyatomic molecules in the gas phase is rapidly becoming a popular tool in the hands of the molecular spectroscopist. In this technique, the photocurrent flowing within a pulse-irradiated sample is monitored as the wavelength of the laser is scanned; photocurrent peaks arise whenever the absorption of an integral number of photons results in a resonance excitation in the target molecule.' A typical experimental setup for MPI spectroscopy is shown in Figure 1. In the simplest experiment, 5-ns pulses from a dye laser pumped at ca. 10 Hz are focused with a 15-cm lens to a 100-pm spot between a pair of flabplate electrodes within the sample cell. With 50-pJ pulses from the dye laser, peak powers of ca. 100 MW/cm2 are generated at the focus, and with a bias of 200-800 V on the plates, dc currents of ca. A can be read with an electrometer or picoammeter. Sample gas is typically at 1-50 torr pressure in such static experiments. As is readily imagined, there are many variations which can be played on this simple experiment: (a) With proper geometry and fill gas, the ionization cell can be run as a proportional counter with high gain. (b) The absorbing molecule may be pressurized to ca. 50 atm with a second gas in a high-pressure ionization cell. (c) The laser polarization may be either linear or circular. (d) Ionization may take place within a mass spectrometer, with subsequent mass analysis of the ionic fragments. (e) The sample may be cooled by seeding into a free-expansion jet or molecular beam. (f) Ionization may be achieved by using two or more lasers of different wavelength, with a variable time separation. (g) The kinetic energy of the photoejected electrons may be analyzed. (h) The MPI experiment may be performed with a liquid sample. All of the above are reported in the literature. How can a visible light pulse ionize a molecule having an ionization potential perhaps three times larger than the energy of one such photon? In general, at some unspecified frequency Y in the visible/UV, such a laser pulse will drive a molecule from the ground state 9oto ionization (9,) via a three-photon coherent absorption. This proceeds 'Permanent address: Department of Chemistry, Bar Ilan University, Ramat Gan, Israel. 0022-3654/82/ 2086-4096$0 1.2510
I
(Moj - hv)(Mok - 2hv)
I
where I is the instantaneous optical power and AEojis the energy separation between stationary levels 0 and j . The factor given above is readily generalized to the case of n-photon absorption. If the laser frequency is scanned, eventually an m-photon resonance will be acheived, Mom - mhv goes to zero, and the overall ionization rate then increases by several orders of magnitude. Thus, on scanning, an ionization spectrum results in which the peaks correspond to m-photon resonances in the absorbing system. The P dependence of the ionization rate, with n larger than m and typically 3 or 4, makes pulsed laser ionization the only practical mode, for in this case I is photons/(cm2 s), whereas for an CW source of the same bandwidth, I is only 10l6photons/ (cm2 s) at best. The lo-' duty cycle of the pulsed source is more than compensated by the far larger value of I and the extremely nonlinear dependence of the ionization rate on I. The MPI technique can be used to several ends. In the hands of the molecular spectroscopist this simple technique has revealed many two-photon excitations in high-symmetry molecules which were otherwise unobservable in one-photon absorption spectra. Moreover, the technique uncovers resonances in the vacuum-UV region (below 2000 A) without the cumbersome vacuum-UV hardware otherwise needed, and the symmetry of the resonance state occasionally can be deduced from the ratio of ionization currents produced by linearly and circularly polarized light. The high sensitivity and specificity of MPI spectroscopy suggests its use as an analytical and indeed, singlemolecule sensitivity has been ~ l a i m e d . ~ . ~ (1)P. M.Johnson, Acc. Chem. Res., 13, 20 (1980). (2)D.A. Lichtin, L. Zandee, and R. B. Bernstein in "Lasers and Chemical Analysis", G . M. Hieftje, F. E. Lytle, and J. C. Travis, Eds., Humana Press, Clifton, NJ,1980, p 1. (3) C. Klimcak and J. Wessel, Appl. Phys. Lett., 37,138 (1980). (4)V.S.Antonov, V. S. Letokhov, and A. N. Shibanov, Opt. Commun., 38, 182 (1981).
0 1982 American Chemical Societv
The Journal of Physical Chemistry, Vol. 86, No. 21, 1982 4007
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E
I G / Figure 1. Experimentalsetup for MPI spectroscopy: A, pump laser; B, dye laser; C, focusing lens; D, glass ionization cell with quartz windows; E, regulated voltage supply; F, electrometer; G, recorder.
Nonlinear Photochemistry Another MPI facet of potential interest is that of multiphoton-driven photochemistry. Resonant excitation in MPI is overall incoherent generally, meaning that an intermediate resonance state of the parent molecule is excited at modest optical powers with a relatively long lifetime. During this long lifetime (up to a few nanoseconds) photochemical transformations (fragmentation, isomerization) of the neutral, parent target molecule may take place which may either quench the ionization or produce a new species which then becomes the MPI target molecule within the laser pulse. In regard to the latter situation, MPI can be much like flash photolysis spectroscopy, in that the front end of the laser pulse can photochemically generate an unstable species, and the back end of the pulse then probes the energy levels of the newly formed entity. On the other hand, ionization of a polyatomic molecule may be but the first step in a photochemical chain, for the parent ion or its ionic decomposition products also can absorb light from the initial laser pulse and then undergo further dissociation. At high optical power, MPI is photochemically degrading, leading eventually to monatomic ions. There also is evidence that, at lower power, photochemical channels can open in MPI which are otherwise closed in one-photon absorption or electron impact excitations. There are several routes a molecule may follow to ionization in MPI. However, in all cases, ionization and/or photochemistry are the result of a single molecule absorbing many photons from the UV/visible laser pulse (up to 10 or more), and so the yield of ionic product will have a strongly nonlinear dependence on light intensity. This we call “nonlinear photochemistry”. Actually, in the regime of high optical power, saturation of certain steps of the ladder climbing may occur with the result that the ion current varies more nearly linearly with optical power. However, even in this narrow region of apparent linearity, the photochemistry is being driven by multiphoton processes. The significance of nonlinear photochemistry rests in the fact that though it may be driven by a total energy of say 20 eV (five 4-eV photons, for example), the photochemical consequences are not necessarily those produced by a one-photon, 20-eV excitation. The differences are due to the stepwise nature of the MPI phenomenon and the competition between absorption and fragmentation processes a t each step. In a new field such as multiphoton photochemistry, a large number of chemically interesting questions immediately present themselves:
(i) Which ionic species are produced? (ii) Which neutral species are produced? (iii) In what order do the myriad photoproducts appear? (iv) Which states of which species are excited, and with how many photons? (v) What are the optical power and wavelength dependencies of the above? (vi) How do multiphoton and one-photon photochemistries compare at the same mean energies? (vii) What are the lifetimes of the fragments in the laser pulse? In this paper, we try to construct some general answers to questions such as these. Several rather obvious facts about the MPI technique should be pointed out. (i) So that some understanding of the photochemical events occurring during MPI can be obtained, it is exceedingly useful to perform mass analysis on the positively and negatively charged ionic products. This is usually done with a time-of-flight mass spectrometer. The determination of the MPI resonance spectrum in general does not require mass resolution, though this can be very helpful, the MPI spectra reported in this paper were determined with flat-plate ionization cells, and hence are not mass resolved. (ii) MPI studies by definition are restricted to investigations of the ionic products of the photochemical reaction, and their yield may be only a small fraction when compared to those processes which produce neutral products. On occasion, identification of the ionic fragments formed in MPI allows one to infer the paths of the neutral-molecule photochemistry which preceeded ionization. (iii) Because all of the spectroscopy in MPI is performed within the span of one laser pulse (ca. 5 ns), the resonance frequencies can be used as fingerprints to identify absorbing species having short lifetimes. (iv) The reciprocal of the collision frequencies in the gas phase will be longer than the laser pulse width at pressures below 10 torr. Thus it is easy to work under conditions which are essentially unimolecular, while at the 5ame time, the effects of collisions can be introduced by a modest rise in pressure. Note, however, that collisions following the laser pulse may alter the ultimate nonlinear photochemistry though not the MPI spectroscopy.
A Classification Scheme In this paper, we describe the three most distinct types of nonlinear photochemical behavior, and then go on to illustrate them with MPI data on a number of organic, inorganic, and organometallic compounds. The three general types of nonlinear photochemical behavior are shown schematically in Figure Z5 In this general scheme, we postulate that the first excitation step is a two-photon coherent absorption from the ground state (Grnd) to the resonance state (Res) via a virtual level (Virt) in the polyatomic molecule MXYZ. While the lifetime in the virtual level (ca. s) is too short to allow photochemical transformation, that in the resonance state is longer; if it is in the range 10-3-1 ns (as compared with a laser pulse width of ca. 5 ns), the molecule might readily decompose or rearrange within the duration of the laser pulse. Thus at the resonance state there is a branching into class A and class B systems, depending on whether the molecule absorbs one or more photons and ionizes or fragmentslrearranges before another photon can be absorbed. The n-photon absorption rate between levels x and y is InuXywhere I is the optical power and ux,, is the relevant cross section. In class A systems, the one-photon ab(5) A diagram of this sort also has been presented by V. S. Antonov and V. S. Letokhov, Appl. Phys., 24, 89 (1981).
The Journal of phvsical Chemistty, Vol. 86, No. 21, 1982
4090 L"
-
-
t
OL
CLASS C
MXY Z -G
r nd
Flgure 2. The three paths of photochemical behavior under multiphoton excitation. Vibrational structure in the resonance state may be either discrete (left-hand side, class A behavior) or continuous (right-hand stde, class B behavior).
sorption rate from the resonance state to a superexcited state SE, l a m , in the ionization continuum is much larger than the decomposition rate y, so that the molecule is preferentially excited to the superexcited level and then relaxes to the parent ion MXYZ+. A t lower laser powers (ca.105 W/cmz> often only MXYZ+ is formed initially, and it does not further photofragment. However, in intense pulses with power of ca. lo9 W/cm2, MXYZ+ may be further excited and fragmented, eventually forming M+, X+, etc. The general course of class A behavior is ionization followed by fragmentation, Figure 2. In a class B system, the photochemical relaxation rate y in the resonance state is much larger than IamE, and so the molecule fragments before another photon can be absorbed. If the molecular fragments are also multiphoton (or one-photon) resonant with the incident light and are no more photustable than the parent system, then they will continue to absorb and fragment until only atoms remain. These atoms, as typified by M in Figure 2, then are multiphoton ionized via atomic resonance states. Let us consider the absorption of the initial three photons. At higher optical power, the number of ions formed NToT equals the number of ionizable species in the focal volume NG, diminished by the fraction of these molecules which choose to fragment in the resonance state, y / ( y + lamE). Thus, NToT = N G [-~y / ( y + ZamE)]. At low power, the expression is more complex, as it involves the laser pulse length, ZUGR,and the spontaneous emission rate from the resonance level to the ground state.6 MPI studies in rare-gas atoms' show as well that a system can reach ionization without having come to resonance anywhere on the ladder. This process is typical of a class C system, and involves the coherent absorption of many photons to instantaneously populate a very high superexcited state (vSE), Figure 2. This state may be much like that attained following one-photon absorption (6)G. J. Fisanick, T. S. Eichelberger, IV, B. A. Heath, and M. B. Robin, J. Chem. Phys., 72,5571 (1980). (7)G. Mainfray and C. Manus, Appl. Opt., 19,3934 (1980).
Gedanken et ai.
in He I photoelectron spectroscopy, and similar consequences would follow. This we call the big bang. In the big bang, autoionization would involve various of the occupied MO's, and the various resulting ionic states may or may not fragment. The ionizing channels in a class C system can produce electrons of very high kinetic energy. In competition with this, the superexcited state may fragment to neutrah8 The class A and class B characteristics described above follow from the relative rates of dissociation and absorption in the molecular resonance state. It is also possible that the absorption rate is larger than the dissociation rate at the two-photon resonance level, but that at the superexcited level the dissociation rate is larger than that for autoionization. In such a case, even though the character of the resonance state predicts class A photochemical behavior, many class B characteristics will appear in the MPI photochemistry (atomic resonances, etc.). Because y is now larger than the autoionization rate ki, rather than larger than the absorption rate la,this situation is given a distinct but related classification, B'. Such B' behavior will resemble closely that in class B molecules. While mwt materials investigated to date are either class A or class B, examples are also known of systems which are class A in one spectral region, and class B in another, and if y is approximately equal to ZuRSE,then mixed behavior can be expected as well. It may also happen that a parent molecule behaves as a class B system to yield a polyatomic neutral fragment, which then behaves as a class A system. Class C behavior has not been identified positively in a molecular system, being known in rare-gas atoms only. Note, however, that even in sharp-line MPI spectra there always is observed an underlying continuum of ionization which in many cases appears to be nonresonant and therefore corresponds to class C. In general, the class A molecules are multiply bonded organics of low-atomic-weight atoms, i.e., acetaldehyde, benzene, butadiene, cyclopropane, etc. Class B, on the other hand, encompasses inorganic and organometallic compounds having one or more heavy-metal atoms, as in CI(CO)~, Mn2(CO)lo,ferrocene, SnCl,, etc. It is apparent that multiphoton excitation of inorganic and organometallic systems can be a clean source not only of metallic ions in the gas phase but of ground- and/or excited-state atoms as weL9 The various atomic states produced upon multiphoton irradiation of class B compounds as determined by MPI are summarized in Table I. (8) W. P. Jesse and R. L. Platzman,Nature (London),195,790(1962). (9)See, for example, M. S. Chou and T. A. Cool, J. Appl. Phys., 48,
1551 (1977). (10)D. P. Gerrity, L. J. Rothberg, and V. Vaida, Chem. Phys. Lett., 74,1 (1980). (11)G. J. Fisanick, A. Gedanken, T. S. Eichelberger, IV, N. A. Kuebler, and M. B. Robin, J. Chem. Phys., 75,5215 (1981). (12)L. J. Rothbera. D. P. Gerrity, and V. Vaida, J. Chem. Phys., 74, 2218 (1981). (13)P. C.Engelking, Chem. Phys. Lett., 74,207 (1980). (14)S.Leutwvler. U.Even. and J. Jortner. Chem. Phys. Lett., 74.11 (1980). (15)J. Bowden and M. B. Robin, unpublished. (16)A. Gedanken and M. B. Robin, J.Chem. Phys., 76,4798 (1982). (17)W.A. Young, M. Y. Mizra, and W. W. Duley, Opt. Commun., 34, 353 (1980). (18)W.A. Young, M. Y. Mizra, and W. W. Duley, J. Phys. E , 13,3175 (1980). (19)C. B. Collins, B. W. Johnson, M. Y. Mizra, D. Popescu, and I. Popescu, Phys. Reu. A, 10,813 (1974). (20)M. Y.Mizra and W. W. Duley, J . Phys. E , 11, 1917 (1978). (21)C. B. Collins, S. M. Curry, B. W. Johnson, M. Y. Mizra, M. A. Chdehmalzadeh, J. A. Anderson,-D.Popescu, and I. Popescu,Phys. Reo. A, 14, 1662 (1976). (22)M. Y.Mizra and W. W. Duler, Proc. R. SOC.London, Ser. A, 364, 255 (1978). (23)M. Y.Mizra and W. W. Duley, Opt. Commun., 28, 179 (1979).
The Journal of Physical Chemistry, Vol. 86, No. 21, 1982 4099
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TABLE I: Class B Production of Ground- and Excited-State Atoms Detected by MPI atom source state A E . cm-' coherence 1 t 2 0 TiCl,, TiBr, Ti
V Cr
c;(co j6, Cr(C0)3C6H6, Cr(C6H6)2
Mn
Fe Ni
co
Mo W Hg
I Ga Pb
Bi
cs
Rb Sn
Si
In Yb
170 387 1255 1 5 975-16 267
1 t 1 t 1t I t
0 7593 7751-8307 20517-20520 21 841 24 058 24 200 24 282-24 303 3 1 048-31 055 0 17 052 23 297 24 779 25 266 27 202 30 354 0-978 6928-8155 880 1332
1 t 1;2 t 1 ; l 1t 1 ; 2 t 1 1 t 1 ; 2 t l 1t1 1t1 1t1 1 t 1 1+1 1t1 l t 2 ; 2 t 1 1t 1 ; 2 t 1 1t 1 ; 2 t 1 1t1 1+1 1t1 1t1 2 t 1 2+ 1 2+ 1 2 t 1
0 1 0 768 0-4830 37 645 3 9 412 44 043 54 069 0 7603 24 788 21 457 35 287 0 29 467 32 588 33 1 6 5 0 111 7 8 11 732 14 499 14 597 0 1 2 816 0 1692 3428 8613 77 223 6299 24 372 24 751 25 068
2 t 2t 2+ 1t 1t 1 t 1t 2 t 2+ 1t 1t 1t 2 t 1t
Distinguishing Characteristics The various spectroscopic and photochemical properties which distinguish class A, B, and C systems from one another are summarized in Table 11. No mention is made of class B' behavior in the table unless it differs from that of class B. Let us now discuss briefly each of the points listed in the table. Point 1. This is a general statement of the photochemical behavior within the three classes and follows directly from the competition between y and IumEin the resonance state. In a class A system, the molecule is ionized to form the parent ion (or a fragment ion), because I u ~ > E y. In a class B system, fragmentation to atoms occurs first (y
ref
this work
2 2 1 1
+2
1 1;lt 1 1;lt 1 1 1 1 1 1 1 1 1 1 1 1
this work 10,11
12
13,14 14
this work this work 15
this work
16 17 18
this work 18
1t1 1+1 1t 1 1t1 1t1 2 t l 1t1 2 t 1 2+ 1 2 1 1 1t 1 2 t 1 2 t 1 2 t 1 1t1 1t1 1t1
19,20
21
this work
this work 22 23
> laRSE), and these subsequently are ionized. In a class C system, fragmentation and ionization are simultaneous rather than sequential and follow the coherent absorption of many photons. Point 2. At low optical power, a class A system is ionized to produce the parent ion. As the power is increased, the parent ion is photolyzed to produce molecular fragment ions, and at the highest powers the fragments are further cleaved to atomic ions. In parallel with the fragmentation rules deduced for electron-impact mass spectra," Pandolfi et al.25point out that fragmentation in the MPI of a class (24)
D.P.Stevenson, Discuss. Faraday
Soc., 10, 35 (1951).
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Gedanken et al.
TABLE 11: Characteristic of Molecular Systems Undergoing Multiphoton Ionization class A class B
class C
1. system is ionized, then atomized
1. system is atomized, then ionized
1. system is atomized and ionized
2. produces MXYZ' a t low power, giving way to MXY', MX', and finally M' a t high power 3. MPI shows molecular resonances characteristic of target molecule 4.MPI resonances originate at ground state only 5. low-power index = IP/hv 6. one-photon spectrum t o resonance state is discrete 7. total energy of the fragments lies above the resonance state 8. ejected electron KE results from molecular ionization process; KE < h u 9. at very high power, the course of ionization is unchanged 10. molecular resonance always Rydberg
2. produces only M' a t all powers
simultaneously 2. produces constant ratios of various ions at all powers
3. MPI shows atomic resonances independent of target molecule 4. MPI resonances originate at ground and also a t excited states 5. low-power index >> IP/hua 6. one-photon spectrum t o resonance state is continuous 7. total energy of the fragments lies below the resonance state 8. ejected electron KE results from atomic ionization process; KE < hv 9. at very high power class B may become class Ad 10. molecular resonance valence shell, or infrequently Rydberg
3. shows no resonances
4.shows no resonances 5. power index >> IP/hv at all powers 6. one-photon spectrum is irrelevant 7. total energy of the fragments lies below the final state 8. ejected electron KE results from molecular ionization process; KE >> hu 9. products independent of power
10. molecular resonances irrelevant
a The power index is larger by 1 in a class B' system than it would be if the same system behaved as class B. cannot be converted t o class A a t high power.
A parent ion will occur so as to place the positive charge on the larger fragment. In contrast, in a class B system, only M+ is produced at all powers, even though the MXYZ+ parent ion may be a stable species (however, see point 9). The MPI mass spectrum of a class B molecule will differ dramatically from that generated by electron impact or one-photon photoionization. Constant proportions of all ions are produced in a class C system regardless of power. Point 3. The MPI spectrum of a class A system will display resonances characteristic of the target molecule, whereas a class B system will display resonances characteristic of constituent atoms but otherwise independent of the target molecule. The patterns of atomic resonances may not be identical however within a group of class B systems containing a common atom, because the molecular photodissociations yielding the atom may give different amounts of the various excited atomic states, depending upon the chemical constitution of the molecular precursors. According to the arguments of Jesse and Platzman,* the resonance intensities in a class B system may be more isotope dependent than those in a class A system. There are no enhanced resonances in the MPI spectrum of a class C system, though certain of the steps may be resonant. Point 4. All of the MPI resonances originate in the ground state of the target molecule in a class A system, whereas they originate at ground and excited levels of the resonant atomic species in a class B system. In certain class B systems, the resonances may originate exclusively at excited levels of the atoms. Point 5. At low optical power, the total-ioncurrent will vary as the nth power of the incident optical flux. For class A systems, the power index n will equal the minimum number of photons, n A , required to ionize the target molecule. On the other hand, in class B, nBis equal to the number of photons required to fragment the system to atoms plus the number required to ionize a particular component atom. In general nB >> nAand nB' = nB + 1. nc is also larger than n A and is the same for all ions. In all cases, the power indices decrease as the optical power is raised, and can even go below 1 for the total ion current (25)R. S. Pandolfi, D. A. Gobell, and M. A. El-Sayed,J.Phys. Chem., 85,1779 (1981).
Class B'
and negative for intermediate fragment ions. The totalcurrent power indices are determined experimentally from the limiting slope of the log (optical power) vs. log (ionization signal) plot at the lowest possible power. In a class B system, measurement of the power index will be complicated by the ac Stark broadening and shift of the resonance line induced by the laser field." For a sharp-line atomic resonance, this effect acts to reduce the true power index by 1-3 units at power levels of ca. lo5 W/cm2, when at resonance or at the low-frequency side of resonance, but increases it greatly when on the high-frequency side.26 Point 6. Because the molecular resonance state in a class A system is stable toward dissociation, the one-photon transition to this state will be rather sharp vibronically, whereas in a class B system the molecular resonance state Res is rapidly dissociating and, in general, the corresponding one-photon Grnd Res transition is continuous or strongly predissociated. A vibronic lifetime of 5 X s or less is sufficient to give an uncertainty width to vibrational features 800 cm-' apart such that the resulting Franck-Condon envelope is continuous. This corresponds to y L 2 X 10l2s-l. On the other hand, the observation of MPI resonances on the nanosecond time scale of the laser pulse implies that the up-pumping rates from the resonance states are larger than lo9 s-l. However, given a power of lo9 W/cm2 and a very large a of 10 Mb, the up-pumping rate l a still will amount to only 10" s-l. Thus in such broad bands y I lola, and class B behavior is to be expected, provided the broadening is due to dissociation and not spectral congestion arising from low-frequency vibrational modes or a mixture of conformers. Note, however, that in molecules of high symmetry, the level reached in the one-photon spectrum may not be that attained with two photons for symmetry reasons. In such a case, the one-photon band profile will be irrelevant to the study of the two-photon excited resonance state photochemistry. No comment can be made about class C systems, since resonance states are not a factor here. Point 7. Of course, if a system were to show class B behavior, then the total energy of the fragments must lie below the total energy of the molecular resonance state. In a class A system, this may be true as well, but with a
-
(26) C. E. Otis
and P. M. Johnson, Chem. Phys. Lett., 83,73 (1981).
Feature Article
barrier between the excited state and its fragments which acts to reduce y. In a class A system, the energy of the fragmented configuration also may lie aboue that of the resonance state. Point 8. The kinetic energy of the ejected electron is a clear indicator of the course of ionization/fragmentation i8 molecular MPI processes. In a class A system, the parent molecule is ionized in an nA-photon process, and the excess kinetic energy carried off by the electron is simply nAhvwhere IP,1 is the ionization potential of the parent molecule. In this case, nAis the power index as determined at low power, and it is assumed that there are no internal excitations of the molecular ion so formed. For a class B system, the electron kinetic energy is similarly given by mhv - IP,, where m is the number of photons required to ionize the atom of ionization potential IPabm. Note, however, that m # nB;for both class A and class B systems, the number of photons nAor m is the minimum number required to ionize the molecule/atom, and the electron kinetic energy will be equal to or less than that of one photon. In contrast, the kinetic energy of an electron ejected from a class C molecule can have an energy much larger than that of a single visible/UV photon, Le., 10 eV or so as occurs in the He I photoelectron spectrum. Point 9. Because laRsEis larger than y in a class A system, going to very high optical power (large 0 will not alter the course of the ionization/fragmentation in such a molecule. A similar statement holds for class C. However, in class B where y is larger than I u ~ at E low power, it is conceivable that, with I sufficiently large, the molecule can be ionized before it fragments, thereby changing character from class B to class A. One might also find that as the one-photon frequency is increased in a class A system and the two-photon excitation of the resonance state occurs at higher and higher energy, y may also increase, thereby converting class A nonlinear photochemistry into class B. Point 10. Without exception, a valence-shell resonance state in the vacuum ultraviolet (wavelength shorter than 2000 A) will fragment so quickly due to its strongly antibonding nature that class B behavior is assured. On the other hand, if the optical electron instead is placed in a nonbonding Rydberg orbit, y usually is then small, and class A behavior results. This factor is irrelevant in a class C system. The distinctly different photochemical behavior of vacuum-ultraviolet excited valence shell and Rydberg states explains why the excited state of a molecule MXYZ* might be unstable (the excited state is valence shell), whereas ita parent ion MXYZ+ is stable even though it has a higher energy (the chemistry of the ion closely resembles that of the corresponding Rydberg states). The point-10 distinction between Rydberg and valence shell behavior holds in so many cases that one is justified in describing the study of polyatomic molecular MPI resonances as a “Rydberg spectroscopy”. Note, however, that in an open-shell positive ion the valence shell excitations will be shifted strongly to lower energies as compared with the neutral molecule, so that one-photon resonances become possible, while at the same time the Rydberg excitations shift upward in energy by about a factor of 4 and so might require 8 photons for resonance. Thus all further resonances in the parent ion or ionic fragment are undoubtedly valence shell.
The Journal of Physical Chemistry, Vol. 86, No. 21, 1982 4101
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terminating at the (n, 3s) Rydberg level; n 3s is a well-known one-photon transition in this molecule. These simple observations (points 3,6, and 10) firmly place acetaldehyde in class A. In accord with this, the mass spectrum of acetaldehyde ionized at low laser power shows a preponderance of CH3CHO+parent ion; however, with increasing laser power, this gives way to CH3CO+,CH3+, CH2+,etc. (point 2). At low power, I, the ionization signal, S, obeys a power law of the form S a P with n = 3, while three photons indeed are required to ionize acetaldehyde 3s resonant wavelengths (point 5). Acetat the n aldehyde is the prototypical class A system. To elaborate somewhat on the situation in acetaldehyde, the first two photons are absorbed in a coherent excitation to (n, 3s) [probably with (n, a*)functioning as the virtual level], and a third photon then converts the (n, 3s) resonance state into the parent ion. This is called a 2 1 process. Absorption of a fourth photon by the parent ion at high optical power then leads to the photochemical production of the fragment ions CH3CO+and HCO+. The first of these fragments can absorb a fifth photon and dissociates to CH4+,CH3+,and CH2+. Among these, CH4+ can be further photolyzed to produce CH3+ and CH2+, while CH2+can be photolyzed to yield CH+. Each of the ionic species listed above is produced with its own characteristic power index. The total-ion power index as conventionally determined with an ionization cell is thus a complex average of these indices and has no meaning by itself, except at very low optical power where only the parent ion is formed. When excited at 2485 A, the acetaldehyde excitation is one-photon resonant with the smooth, continuous highfrequency wing of the n ir* valence transition, and, by point 6, it then follows that the nonlinear photochemistry in this region will be class B, as observed.29 The MPI spectrum and photochemistry of Cr(CO)6in the 4000-5000-A r e g i 0 n ~ ~ Jstand l 9 ~ ~ in strong contrast to those of acetaldehyde discussed above. The one-photon spectrum of Cr(CO)6 is broad and continuous at all wavelengths shorter than 3500 A (point 6), whereas the MPI resonances in this molecule are atomically sharp and plentiful. These resonances are found in the MPI spectra at identical wavelengths regardless of which Cr compound is irradiated (point 3), and, moreover, the MPI-driven mass spectrum of Cr(CO)6 reveals only a single ion, i.e., Cr+ (point 2). This latter observation stands in strong contrast to the conventional situation in which the mass spectrum is generated by electron impact, for in this case over a dozen ions are observed, from Cr(CO)6+to Cr+.31 Kinetic-energy analysis of the MPI-ejected electrons shows that in Cr(CO)6they originate in the ground and excited states of the Cr atom (point 8). Finally, at low power, a power index of 5-6 is observed, although only three photons are required to ionize Cr(CO)6itself (point 5). Clearly, Cr(CO)6 in this region is exhibiting class B behavior. To summarize the situation in Cr(CO),, the first two photons are coherently absorbed to excite the molecule in the broad-band region. This results in immediate dissociation to Cr(C0)5,which then absorbs another two photons [probably incoherently, i.e., 1 + 11 and dissociates to produce Cr(1) in its ground and excited states. These
-
+
-
(27) B. A. Heath, M. B. Robin, N. A. Kuebler, G. J. Fisanick, and T. S. Eichelberger, IV, J. Chem. Phys., 72, 5565 (1980). (28) T. S. Eichelberger, IV, and G. J. Fisanick, J. Chem. Phys., 74, Two Examples 5962 (1981). (29) J. P. Riley and K. L. Kompa, Adu. Mass Spectrom., 8B,1800 Let us now consider two examples which are already in (1980). the literature but which illustrate many of the points listed (30) M. A. Duncan, T. G. Dietz, and R. E. Smalley, Chem. Phys., 44, in Table 11. In the region between 3600 and 3700 A there 415 (1979). is a sharp two-photon resonance in a ~ e t a l d e h y d e ~ 9 ~ ~ ~(31) ~ ~G. A. Junk and H. J. Svec, 2.Naturjorsch. E , 23, 1 (1968).
4102
The Journal of Physical Chemistty, Vol. 86, No. 21, 1982
Gedanken et al.
TABLE 111: NIP1 Resonances in HgBr, MPI spectral diff," freq , cm-, cm-' assignmentu
i
?
I
I
7
I
i
I
22 935 24 520 24 710 25 820 25 900 26 020 26 105 27 295
i
I
I
27 360 27 390 27 435 27 835 27 845 27 885 28 100
1
10
130
22 938 24 517 24 705
3Py(6s6p)+ 3S,(6s7s) 3Py(6s6p)-+ 'S0(6s7s) 'P;(6s6p) + %,(6s7s)
27 290 27 293 27 353 27 388
3P0,(6s6p)+ 'D2(6s6d) 'P:(6s6p) + 'D1(6s6d) 3P:(6s6p) 3D,(6s6d) 3P:(6s6p) 3D3(6s6d)
27 836 27 840 27 874 28 092
'Py(6s6p) 3D,(6s10d) 'PY(6s6p) + 3D,(6s10d) 'Py(6s6p) + ' P y ( G ~ l l p ) ? ~ 'P:(6s6p) -+ 'S0(6s12s)
-f
--f
--f
Reference 33. This transition is one-photon forbidden by parity.
POWEQ
Flgure 3. Power law plots for Hg(CH3), and Sn(CH3), in the low-power regime.
-
molecular absorptions are most likely valence shell metal ligand charge-transfer promotions. The Cr(1) atom then undergoes 2 1 excitations to form Cr+. Though more than enough energy is supplied in this seven-photon excitation to form Cr(CO)6+,this ion is not observed.
+
Applications We now consider the course of nonlinear photochemistry in several other organic, inorganic, and organometallic systems. While our choices of target molecules do not exhaust the possibilities, they are broad enough to illustrate that the photochemistry of very different types of chemical systems can be categorized and understood as examples of class A, B, or C behavior. Both transitionmetal and nontransition-metal systems have been considered. Divalent Mercury. Divalent mercury systems are interesting because they show both class A and class B behavior. We have studied the MPI processes in HgY2, where Y = CH3, C1, Br, and I. As already reported,32the MPI spectrum of Hg(CH3), in the one-photon region 4000-3550 A displays very sharp two-photon molecular resonance features, which place it in class A by point 3. In accord with this, the MPI mass spectrum of Hg(CH3), is rich in parent ion at low power (point 2), the power index at low power is 3.0, Figure 3 (point 5), the one-photon absorption in the 2000-1800-A region is sharp (point 6), and no Hg atomic lines are seen in the MPI spectrum (point 3). The two-photon excitation in question in Hg(CH3)2 involves the promotion of a Hg-C a-bonding electron into a spin-orbit component of the 6p7r Rydberg orbital (point 10). At low power in Hg(CH3),, one has a straightforward 2 1 molecular ionization in a class A system. By contrast, in the mercury halides, the lowest onephoton excitation is to an A state, in which a halogen lone-pair electron is promoted into the Hg-X a* MO. This excitation is valence shell (point 10) and is totally structureless in the one-photon spectrum (point 6), thus leading one to expect class B photochemical behavior. Indeed, the MPI spectra of the mercury halides are rich in class B Hg-atom transitions (point 3).
+
(32)A. Gedanken, M. B. Robin, and N. A. Kuebler, Inorg. Chem., 20, 3340 (1981).
The features observed in the MPI spectrum of HgBr, are listed in Table 111. Many of these appear as sharp lines and are readily assigned to resonances in the Hg(1) atom. Note however, that these originate at atomic levels between 37 645 and 54069 cm-' above the 'So ground state of Hg(1); moreover, three-photon resonances in atomic mercury vapor are seen in just this spectral region, originating at the 'So ground state,32but these are not observed when HgBr, sewes as the source of Hg(1). Thus the multiphoton dissociation of HgBr2in the 4300-3550-A region yields only excited Hg(1) atoms. Many of the MPI lines originate at 'P, which is 54069 cm-' above the ground state.33 Ionization of resonances originating at this state are then (1 1) processes. According to L ~ r i othe , ~ 'P1 ~ level at 54 069 cm-' has a radiative lifetime of 1.31 X s. The fact that we see up-pumping from this level means that our absorption rates from 'P1 to higher levels, la, are larger than lo9 s-l, but smaller than the maximum absorption rate of ~ O ' ' S - ~ . Could some of our atomic excited states be the result of decay from even higher excitations? Probably not, since these higher states (7'D, 8lS, lO'P, etc.) have lifetimes of 100 ns and up,35and so are not radiatively active during the 5-11s laser pulse. Two-photon excitation of HgBr, by a fundamental wavelength in the vicinity of 4400 A should populate the A state; X A is a transition with a continuous band shape. According to calculations by Wadt,36the X A promotion is immediately followed by dissociation into two Br atoms and the Hg atom in its ground state. Thus Hg-atom MPI resonances resulting from A-band resonance and dissociation should originate at the 'So ground state. This is contrary to our findings. Note, however, that Wadt's assignment of the A band (lZg+ 311,) is to that one state which is one-photon allowed of five predicted in this region. Our experiment involves a two-photon excitation, however, which must be 'Zg+ ' s 3 1 1 g (via an intermediate II, level); according to Wadt's calculation, these IIg states relax to X atoms and the stable ground state of HgX. We presume then that the HgBr(X) next is promoted in a two-photon process to unbound levels which yield excited-state Hg atoms. In
+
-
-
-
(33) C. E. Moore, Nat. Bur. Stand. US.,Circ. No.467 (1958). (34) A. Lurio, Phys. Reu. A , 140, 1505 (1965). (35) F.H.M. F a i d , R. Wallenstein,and R. Teets, J. Phys. E , 13,2027 (1980). (36) W. R. Wadt, J. Chem. Phys., 72, 2469 (1980).
The Journal of Physical Chemistry, Vol. 86, No. 21, 1982 4103
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ONE -PHOTON WAVELENGTH.
a
Flgure 4. The MPI spectrum of Sn(CH,), in the gas phase.
fact, Wadt states that in HgCl there are a "myriad of states that arise from Hg(3p) + Cl(2P) and Hg('P) + Cl(2P) limits. Most of these are repulsive ...",3' and that 'P, of Hg(1) would be formed from HgBr(X) in a transition to an unbound level approximately 7 eV above the ground state.% This is just the two-photon region in which we observe the atomic excitations originating at lP,. Overall, the ionization process HgBr2 Hg+ + 2Br is 2 + 2 + 1 + 1, in terms of coherence. The MPI spectrum of HgC1, shows many but not all of the features discussed above. Thus the Hg-atom excitations from 3P2and from 'P, listed in Table I11 for HgBrz are also seen with HgC12. Surprisingly, the low-energy excitations from 3P1and 3P0are not observed in HgC12. Of course, the molecular excitation in the 25 000-26 000-cm-' region of HgBr2 does not appear in HgC12,except possibly for an isolated feature at 25 796 cm-l. The strong contrast between the nonlinear photochemistries of Hg(CH3)2and HgX2can be traced directly to the different orbital natures of the resonance states and the chemistry that then flows from them. In the case of Hg(CH3)2, the resonance state is Rydberg and stable, whereas in HgX2, it is valence shell and unstable (point 10). Tetravalent Tin. The large differences in the photochemistries of Hg(CH3)2and HgX2 led us to investigate a similar pair, Sn(CH3)4and SnC14. The first one-photon electronic transition in SnC1t9is found centered at 2000 A (50000 cm-l), with a perfectly smooth, symmetric vibronic profile. Compared to the SnC1, ionization potential of 97 500 cm-', the frequency of the 50000-cm-' band is far too low for it to be a Rydberg excitation, and consequently it has been assigned as valence shell.@ It is most likely an A-band transition (chlorine 3p u*), similar to those in HgX2 in the same region. According to points 6 and 10, the MPI of SnC1, should show class B nonlinear photochemistry in the laser pulse.
-
-
(37)W. R. Wadt, Appl. Phys. Lett., 34,658 (1979). (38)W. R. Wadt, personal communication. (39)G. C. Causley and B. R. Russell, J. Electron Spectrosc. Related Phenom., 11,383(1977). (40)M. B.Robin,"Higher Excited States of Polyatomic Molecules", Vol. I, Academic Press, New York, 1974; Vol. 11, Academic Press, New York, 1975.
The lowest band in the Sn(CH3), electronic spectrum has been described as a continuum extending from 2100 to 2600 Nothing else is known of this molecule's electronic spectrum. It would seem that, on the basis of point 6, Sn(CH3), is also class B, in contrast to Hg(CH3)2. A small section of the MPI spectrum of Sn(CH3), is shown in Figure 4. The MPI resonances in Sn(CH3), consist of a very large number of lines, extending from 4100 to 3587 A. As illustrated in Figure 4, many, but not all of these lines are assignable as two-photon resonances in the tin atom, originating at the 3P0ground state and at the 3P1,3P2,and 'D2 excited states (Table I). A large number of other equally sharp and prominent lines also are observed but could not be assigned under any assumptions as either Sn(1) or Sn(I1) transitions according to the levels listed in ref 33. Though the two-photon region spanned by 4100-3587 A in one photon is beyond the continuum absorption described for Sn(CH3),,there is little doubt that we are exciting into a second dissociative continuum above it and that this leads to class B photochemistry in this molecule. When a combination of various data are ~ s e d , 4an ~-~~ energy level diagram can be constructed for Sn(CH3),, its fragments and ions, Figure 5. One sees from this that if Sn(CH3),were to behave as a class A molecule, then at low optical power at wavelengths below 4100 A, Sn(CH3)f+ would form in a three-photon 2 + 1 process. However, if y is larger than la in the resonance state of Sn(CH3),, then it will fragment, and a series of such class B acts in the fragments eventually will lead to the production of Sn+ in a six-photon process. We have measured the power exponent for ionization of Sn(CH3), at 3805 A,and find 5.5 f 0.6 from the data at the lowest measurable powers, (41)H.W.Thompson and J. W. Linnett, h o c . R . SOC.London, Ser. A, 156,108 (1936). (42)J. J. de Ridder and G. Dijkstra, Recueil, 86,737 (1967). (43)J. L. Occolowitz, Tetrahedron Lett., 5291 (1966). (44)B.G. Hobrock and R. W. Kiser, J. Phys. Chem., 65,2186(1961). (45)M.F.Lappert, J. Simpson, and T. R. Spalding, J. Organomet. Chem., 17,P1 (1969). (46)A. L. Yergey and F. W. Lampe, J. Organomet. Chem., 15, 339 (1968). (47)F.W.Lampe and A. Niehaus, J. Chem. Phys., 49,2949 (1968). (48)D. B. Chambers and F. Glockling, Inorg. Chim. Acta, 4, 150 (1970).
4104
Gedanken et ai.
The Journal of Physical Chemistry, Vol. 86, No. 27, 1982 ONE-PHOTON FREQUENCY,
CM-'
25.500
26.000
26,500
27.000
I
I
I
I
sn'
Sll+ A
15 EX
4
EX
Sn(CH3)i CLASS A
Ees IC
SfllCHslj
virt CLASS B
1
39SO
3900
0
Grnc
3 m
3850 OWE-WOTON W N E L E N G T H .
3 m
1
Figure 6. The MPI spectra of SiCI, (A) and Si(CH,), (B), both in the gas phase. The dye gain in this region is shown as curve C.
Figure 3. The difference between the measured power index of 5.5 and the anticipated value of 6.0 can be attributed to the action of the ac Stark effect (point 5 ) . A power index of 5.5 is equivalent to 18 eV at 3805 A, which clearly indicates that a large part of the overall photon energy (even at low optical power) is being used to break bonds in molecular species leading to ionic species. Note also, Figure 5, that several fragment configurations in the Sn(CH,), system have energies at or below that of the parent-molecule resonance state (point 7 ) . The obvious production of Sn atoms in the laser photolysis of Sn(CH3),, Figure 3, implies the presence as well of methyl-radical resonances in the MPI spectrum of this compound. As described in another paper,16such methyl group two-photon resonances have been assigned to two broad features observed at 67 820 and 67 990 cm-' in Sn(CH3)k MPI mass spectrometric measurements on Sn(CH,), are totally in line with the characteristics of class B nonlinear photochemistry. Thus we have excited several of the MPI resonance lines of Sn(CH,), within a TOF mass spectrometer and observed only Sn+ at all optical powers, as expected for a class B system (point 2). In contrast, the ions Sn(CH,),-,+ are found in the electron impact mass spectrum of Sn(CH3),, with x running from 0 to 4.42 We have attempted to observe the MPI resonances of Sn(CH,), using an optoacoustic cell rather than an ionization cell.49 An intense optoacoustic signal was obtained, but it was featureless in the region of intense MPI resonances! Apparently, the heat generated by the multiphoton ionization and recombination is so slight that it is lost within the noise of the heat generated by two-photon absorption followed by the photochemical and photophysical relaxations of neutral species. A brief study of SnC, quickly confirms our expectation that it will show class B photochemistry. This expectation
is based upon the continuous nature of its absorption bands (point 6 ) ,the valence nature of its resonance state (point lo), and its energy-level diagram, which looks much like that of Sn(CH,), (Figure 5), with two fragmented configurations below the resonance state (point 7 ) . The MPI spectrum of SnC1, shows all of the Sn(1) resonance lines observed for Sn(CH3), (point 3). There are no chlorine-atom resonances in our spectral region. The ions resulting from 3550-A irradiation of SnC1, within the mass spectrometer proved to be 80% Sn', 20% SnCl', and less than 1% SnC12+. In the electron-impact (31 eV) mass spectrum of SnC14,50SnC13+is the major ion with all others at about 1/5 its intensity, while with an electron-impact energy equal to the ionization threshold, SnC14+is predominant. The MPI mass spectrum of SnC1, illustrates a hidden assumption within our classification scheme. Referring to Figure 5 as an example, one sees that the appearance of Sn(1) resonance lines depends not only on the class-B behavior at the resonance state of Sn(CH,), but also on the class-B behavior at the corresponding resonance state in each of the intervening molecular fragments. This is the case in Sn(CH,), since no molecular ions are observed; however, in SnCl,, it appears that the resonance transition in the SnCl photofragment is in part class A, since significant SnCP is observed, and in part class B, since significant Sn+ and Sn-atom resonances are observed. SnCl, then is an example of a system which shows class B behavior to a certain point up the dissociation-ionization ladder, followed by a branching into competitive class A and class B channels at that point. Preliminary data suggest that Ge(CH3),and GeC1, also show the mixed class A-class B behavior. The MPI spectrum of Sn(C6H5),in a flat-plate ionization cell also was determined; the MPI signal simply followed the dye-gain curves in the 3580-4140-A region without a trace of an Sn(1) resonance. The unexpected class A behavior of Sn(C6H5),under high-energy irradiation may
(49) M. B. Robin in "Optoacoustic Spectroscopy and Detection", Y.-H. Pao, Ed., Academic Press, New York, 1977, p 167.
(50) A. S. Buchanan, D. J. Knowles, and D. L. Swingler, J. Phys. Chem., 73, 4394 (1969).
Figure 5. Energy level diagram for Sn(CH,),, its fragments, and ions.
The Journal of Physical Chemistry, Vol. 86, No. 21, 1982 4105
Feature Article
23
21
&+3A ~ A L
u+%(-2693259
4
3
2 I
I
CP+2A (-2694587)
CPOtA(2636092)
Flgure 7. The nonlinear photochemical scheme deduced for norbornadiene under laser irradiation at 3550 A. The following abbreviations have been used: NBD, norbornadiene;CPD,cyclopentadiene; A, acetylene; CP,cyclopropene; M, methylene.
relate to the resistance toward fragmentation generally shown by phenyl-containing compounds.51 (51)J. P.Manion and M.Burton, J. Phys. Chem., 56, 560 (1952).
Titanium Halides. We have found TiC1, and TiBr, to be very intense sources of Ti-atom resonances. Class B nonlinear photochemistry in these materials is not unexpected since they have one-photon s p e ~ t r awhich ~ ~ , sug~~
4106
The Journal of Physlcal Chemistry, Vol. 86, No. 27, 1982
gest a class B classification by points 6 and 10. The multiphoton ionization processes were so intense in these compounds that, upon removal of the focusing lens, signals adequate for recording spectra were still present. Between 3570 and 4000 A in the MPI spectrum of TiCl,, there are a great many lines resting upon a continuum which largely follows the laser dye-gain curves. In the region 4000-4530 A,the lines become sparse to nonexistent, but the continuum persists, again following the dye gain. All of the lines observed in TiCl, are also observed in the MPI spectrum of TiBr,. As listed in Table I, class B photochemistry in TiC1, and TiBr, results in the formation of Ti(1) atoms in the a3Fz ground state and in 16267 cm-' above the ground excited states up to z6G6, state. Note that, as with the MPI spectra of the tin compounds, the TiX, spectra contain many lines which cannot be assigned by using Moore's tables.54 There are no C1 or Br atom resonances in the region in which we work. The intense continua underlying the atomic resonances in TiX, most likely result from the class A ionization of molecular resonances. In support of this, the mass spectrum of TiCl, ionized in the sharp-line region 3770-3790 A displayed the large Ti+ mass peak as expected, and also TiCP with 5 1 0 % the intensity of Ti+. Thus at some point in the chain, TiCl, when excited branches to Ti and TiCl,,, with the first of these going on to Ti+, and the second to TiCl+. Because 4 > x > y L 1, we have three possible solutions: x = 3, y = 2; x = 2, y = 1; and x = 3, y = 1. We cannot decide among these. In contrast to the laser MPI of TiCl,, the electron impact (70 eV) mass spectrum of TiCl, shows all possible ionic fragments, with TiC13+the most prominent, and Ti+, TiCl+, Tic&+,and TiC14+each approximately half as abundant.55 The initial two-photon resonances excited in the MPI spectrum of TiC14 fall among the second and third continuum bands of this molecule. These bands have been assigned as valence shell charge-transfer excitations (chlorine 3p Ti 3da* and 3d7r*), and resemble A bands." The continuum revealed in the MPI spectra can appear as such only if all of the neutral species from TiC14to that leading to Tiel+ also have broad continua in the 35704530-A region. Tetravalent Silicon. In general, the chemistry of the group 4 elements are similar, and on this basis one would expect SiCl, and Si(CH3),to behave in a class B manner, as do their Sn counterparts. A possible difference arises when looking at the one-photon spectra of these silicon compounds, for SiC1, has a remarkably short-wavelength spectrum for a tetrahalide,56 with its first band centered at 1400 A. Excitations to this band are beyond our twophoton wavelength range, but will be three-photon resonant. The first absorption band of Si(CH3)457 is centered at 1720 A, and has a long wavelength tail which is just within our two-photon range. The MPI spectrum of SiC1, consists of a forest of sharp lines, Figure 6, a small fraction of which can be assigned to Si atom resonances originating at the 3P1,,P2,and lDp excited levels. The largest number of these obviously atomic lines could not be assigned to Si(I), however. In +
(52)C. A. L.Becker, C. J. Ballhausen, and I. Trabjerg, Theor. Chim. Acta, 13,355 (1969). (53) L. Di Sipio, G. de Michelis, E. Tondello, and L. Oleari, Gazz. Chim. Ztal., 96, 1785 (1966). (54)C. E. Moore, Nut. Bur. Stand. US.,Circ., No. 35 (1971). (55) R. W. Kiaer, J. G. Dillard, and D. L. Dugger, Adu. Chem. Ser., No. 72. 153 (1968). ~.~ ~~- -, (56) G. C. Caueley and B. R. Russell, J . Electron Spectrosc. Related Phenom., 11, 383 (1977). (57)R. Roberge, C. Sandorfy, J. I. Matthews, and 0. P. Strausz, J . Chem. Phys., 69, 5105 (1978). ~~
Gedanken et al.
view of the class B behavior of SnCl,, SiCl,, and Sn(CH3),, it is a surprise to find that Si(CH3),is class A! As is shown in Figure 6, the largest part of the Si(CH3), MPI spectrum is a continuum which follows the dye gain curve, with only a weak suggestion of the strongest Si(1) atomic resonances perched upon this. Methyl Iodide. Methyl iodide is a hybrid system in the sense that it is clearly class A in one spectral region but class B in another. This follows naturally from points 6 and 10 in Table 11,for the one-photon spectrum of methyl iodide in the region 3600-2100 A is continuous, corresponding to an n -+ u* A-band valence shell excitation, whereas the second excitation beginning at 1950 8, is an n 6s Rydberg transition which is quite sharply structured.5s Thus resonance in the first of these should give class B behavior while resonances in the second will behave as class A. Excitations which are one-photon resonant in the A band of methyl iodide yield CH, and I fragments with near-unit quantum yield, with the I atoms in either the 2P3/2 or 2P1/2state. Accordingly, when irradiated at wavelengths shorter than 3100 A,MPI atomic resonances are observed to originate from these two states of the iodine atom, as expected for class B behavior. Resonance features are also observed for the methyl radical.16 On the other hand, pumping at 4000-3300 A sidesteps the n u* resonance, but two-photon excites the n 6s and n 6p molecular levels. Consequently, only the CH31molecular Rydberg resonances are o b ~ e r v e d . ' ~ -The ~ ~ mass spectrum in this region shows a strong parent-ion peak (point 2), together with I+ and CH3+which must come from photodissociation of the parent ion which is onephoton resonant in this wavelength region. Further, the thresholds for the production of CH3+and I+ from CH31+ are exceeded by a one-photon absorption in the parent ion. The class A-to-class B shift of nonlinear photochemistry displayed by methyl iodide is due to a change of resonance state on scanning through the UV/visible region, i.e., from (5p,u*) valence shell to (5p,6s) Rydberg. A somewhat similar change is observed in N02,60where a shift from class A to class B behavior is encountered upon exceeding the dissociation wavelength (4000 A) of the valence excitation in the visible region. Norbornadiene. As a final example, we consider the nonlinear photochemistry in norbornadiene and its isomer +
-
Y
norbornadiene
+
quadricyclane
-
quadricyclane.61 The MPI spectrum of norbornadiene is clearly that of a class A system, showing r2 3s,3p, and 3d Rydberg transitions as two-photon resonances (point 10). A t very low power in the mass spectrometer, C,H,+ and C6H,+ ions are observed weakly, and with increasing (58)R. A. Boschi and D. R. Salahub, Mol. Phys., 24, 289 (1972). (59)D. H.Parker, R. Pandolfi, P. R. Stannard, and M. A. El-Sayed, Chem. Phys., 45, 27 (1980). (60)M. B. Robin, Appl. Opt., 19, 3941 (1980). (61)A. Gedanken, G.J. Fisanick, K. Ragavachari, T. S. Eichelberger, IV, M. B. Robin, and N. A. Kuebler, t o be submitted for publication.
J. Phys. Chem. 1082, 86, 4107-4112
power give way to C3H,+, C2H,+,CH,+, and H+ (point 2). Through a combination of ab initio calculations, photoelectron spectroscopy, and symmetry arguments, the tenphoton fragmentation scheme of Figure 7 was derived for norbornadiene in a laser pulse at 3550 A. The ionic products resulting from nonlinear photochemistry in quadricyclane are very similar to those observed in norbornadiene, presumably from the common intermediate C5H6+in the two systems.
Summary In this work, we have attempted to construct a coherent picture of laser-driven molecular photochemistry using assorted observations on widely different compounds. The end result is the classification scheme and list of characteristics outlined in Table 11. Though it is not yet clear
4107
as to how general (and useful) such a classification scheme will be, it is clear that there are at least three distinct types of nonlinear photochemical behavior. One also sees that the course of one-photon photoionization in the vacuum UV can be very different from that of multiphoton ionization in the visible/UV even though equivalent amounts of energy are deposited in the molecule in the two situations. The stepwise nature of the molecular MPI process is responsible for this difference. Though we observe only ionic end products, a certain amount of information can be gleaned from this concerning the nature of the photochemistry occurring among neutral species. Acknowledgment. We acknowledge helpful conversations with G. J. Fisanick, William Wadt, and T. Bowmer, and the computational aid of Tali Gedanken.
ARTICLES Infrared Spectroscopic Study of Hydroxyl Groups on Silica Surfaces Isao Tsuchlya Electrotechnlcal Laboratory, Umezono, Sakura-mura, Nllharlgun, Ibarakl, Japan 305 (Received: June 9, 198 7; I n Flnal Form: June 17, 1982)
Infrared spectra of Aerosil silica were measured for samples which were subjected to bakeout, deuterium exchangb, and diethyl ether adsorption. The absorbance difference spectrum between the infrared spectra obtained at different conditions showed several peaks and shoulders for each experiment. In consideration of a model of surface hydroxyls, it is concluded that infrared absorption of the silica degassed under vacuum below 500 "C contains a number of contiguous bands and that most of these bands are attributed to various types of hydroxyl species in the model. In an attempt of decomposition of the difference spectra by a curve resolver, these were respectively resolved into several components,which could be related to each other among the three experiments.
Introduction Many investigators have reported studies of hydroxyl groups on silica surfaces by infrared spectroscopy. It had been previously indicated that infrared absorption due to the OH stretching vibration of the surface hydroxyls consists of a sharp band and a very broad one, which had been assigned to isolated and hydrogen-bonded hydroxyls, respe~tively.'-~ Thereafter, the surface hydroxyls have been considered to be classified into more than two molecular species. Only the isolated hydroxyls exist on silica surfaces degassed at very high temperature. It was pointed out that the OH stretching band due to the isolated hydroxyls, when degassed above 800 "C, has a fine structure and is resolved into three component bands by a curve reso1ver,4v5though (1)L. H. Little, A. V. Kiselev, and V. I. Lygin, "Infrared Spectra of Adsorbed Species", Academic Press, London, 1966. (2) M. L. Hair, "Infrared Spectroscopy in Surface Chemistry",Marcel Dekker, New York, 1967. (3) A. V. Kiselev and V. I. Lygin, "Infrared Spectra of Surface Compounds", Wiley, New York, 1975, Israel Program for Scientific Translation, Jerusalem. 0022-365418212086-4107$0 1.2510
objections to this result were brought forward by other For silica treated at lower temperatures the infrared absorption should contain many more bands because of isolated and hydrogen-bonded hydroxyl^.^ The bands observed for silica gel baked out at 600 "C were assigned to four different types of surface hydroxyls.1° The infrared absorption of silica baked out at relatively low temperatures extends from 2.5 to about 3.2 pm and appears to have a smaoth and long tail on either side of the sharp band due to the isolated hydroxyls. But both of the tails are qbite different in shape and moreover the absorption intensities of the tails do not vary uniformly, when the sample is subjected to any treatment such as (4)M. L. Hair A d W. Hertl, J. Phys. Chem., 73,2372 (1969). ( 5 ) F. H. Van Cauwelaert, P. A. Jacobs, and J. B. Uytterhoeven, J. Phys. Chem., 76,1434 (1974). (6)J. A. Hockey, J. Phys. Chem., 74,2570 (1970). (7)B. A. Morrow and I. A. Cody, J . Phys. Chem., 77, 1465 (1973). (8)P.R. Ryason and B. G. Russell, J.Phys. Chem., 79,1276 (1975). (9)S.Kondo, M. Muroya, and K. Fujii, Bull. Chem. SOC.Jpn., 47,553 (1974). (10)A. J. Van Roosmalen and J. C. Mol, J . Phys. Chem., 82, 2748 (1978).
0 1982 American Chemical Society
