J . Phys. Chem. 1984,88, 5110-5112
(b)
u 65 0
700 AE (eV 1 n Figure 1. (a) L3and L2white lines in the deconvoluted EELS of two manganese oxides. L3line is to the left of the figure. (b) Variation of the relative intensities of the L3and L2 white lines of manganese oxides with the number of d electrons. Intensities were obtained from peak areas in deconvoluted spectra after background subtraction. Error bars are indicated only for those points where the squares or circles do not cover the measured uncertainties.
+
of two expected on the basis of the 2j 1 degeneracy. The L3/L2 intensity ratio is maximal for the d5 configuration, and this feature can also be exploited for the purpose of characterization of microcrystalline specimens. The variation of the L3/L2intensity ratio with the d-orbital occupancy can be understood, in principle, on the basis of the J selection rule for the unoccupied d states. The L2 transition probability decreases in going from do to d5, whereas
the L3 transition probability, which is finite at d5, decreases as the d orbital is filled beyond this occupancy. The L2 and L3 edge energies in EELS increase as the number of unoccupied d-electron states (oxidation state of the metal) increases; and such chemical shifts are well d o ~ u m e n t e d . ~The oxygen K edges, in addition to the metal L edges, exhibit features which are sensitive to the occupancy of the d orbitaL6 But what is of especial merit about EELS studies (by electron microscopy) of transition-metal systems is that we are able to retrieve such revealing information from minute quantities (IO-’* to g) of powdered material, the structure and chemical composition of which can be ascertained (by electron diffractionlo and EELS intensityll data) simultaneously. Furthermore, under favorable circumstances plasmon loss peakss associated with EELS can also be of considerable value in characterizaton of materials: and electron Compton scattering profiles,12which are relatively easily recorded, can yield information about bonding in certain simple systems-such as in amorphous carbon, where the hybridization has been shown13 to be predominantly sp2.
Acknowledgment. We thank the Royal Society and the S. E.R.C. for support. C N R R is thankful to the University of Cambridge for the Jawaharlal Nehru Visiting Professorship. (9) G.Sankar, P. R. Sarode, and C. N. R. Rao, Chem. Phys., 76, 435 (1983), and references therein. (10) J. M. Thomas, Ultramiscroscopy, 8, 13 (1982). (11) R. F.Egerton, Phil. Trans. R.SOC.London, Ser. A , 305, 521 (1982). (12) B. G . Williams and J. M. Thomas, Inf. Reu. Phys. Chem., 3, 39 (1983). (13) B. G.Williams, T. G . Sparrow, and J. M. Thomas, J . Chem. Soc., Chem. Commun., 1433 (1983).
Picosecond Excited-State Absorption of Alkyl Nitrobenzenes in Solutiont R. W. Yip,* D. K. Sharma,* Division of Chemistry, NRCC, Ottawa, Ontario, Canada, K I A OR6, and Canadian Picosecond Laser Flash Photolysis Centre, Concordia University, Montreal, Quebec, Canada, H3G 1 M8
R. Giasson,l and D. Gravel* Ddpartment de Chimie, Universitd de Montrdal, C.P. 6210, Montreal, Qudbec, Canada, H3C 3Vl (Received: August 13, 1984)
Several alkyl nitrobenzenes in solution were studied by picosecond transient spectroscopy. Two transient absorption bands in the 400-650-nm region were observed and are assigned to the n,n* excited triplet state. The energy of the triplet state of nitrobenzene was determined to be 58 kcal mol-’. Rapid intersystem crossing is indicated by the 15-ps rise time measured for nitrobenzene, Intramolecular hydrogen abstraction occurs from the n,n* triplet state with a rate constant of 0.9 X lo9 for o-nitroethylbenzene and 50.1 X lo9 s-l in o-nitrotoluene.
Since the discovery of the photochemical isomerization onitrobenzaldehyde to o-nitrosobenzoic acid by Ciamician and Silber in 1901,l nitrobenzene and substituted nitrobenzenes have been found to undergo a variety of reactions,2 including intermolecular3 and intramolecular benzylic4 and homobenzylicZb hydrogen abstractions, photoaddition to olefin^,^ photoredox in aqueous solution: and photosub~titution.~Among these, the general reactivity of o-nitrobenzyl systems to display intramolecular hydrogen abstraction has been exploited to great advantage tNRCC No. 23772. Presented, in part, at the Xth IUPAC Symposium on Photochemistry, July 22-27, 1984, Interlaken, Switzerland. *Canadian Picosecond Laser Flash Photolysis Centre, Concordia University, Montreal. Holder of an NSERC scholarship 1983-85.
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in the development of a number of photochromic compounds,s photolabile protective groups: and more recently photoresists.1° (1) Ciamician, G.;Silber, P. Chem. Ber. 1901, 34, 2040; 1902, 35, 1992. Morrison, H. A. In “The Chemistry of the Nitro and Nitroso (2) Groups , Feuer, H., Ed.; Interscience: New York, 1969; Part I, Chapter 4, p 165. (b) Dopp, D. Top. Current Chem. 1975,55, 49, and references cited
p)
therein.
(3) Hurley, R.; Testa, A. C. J . Am. Chem. SOC.1968, 90, 1949. (4) (a) de Mayo, P. In “Advances in Organic Chemistry”, Raphael, R. V. A., Taylor, E. C., Wynberg, H., Ed.; Interscience: New York, 1960; Vol. 2, p 367. (b) de Mayo, P.; Reid, S . T. Q.Reu. Chem. SOC.1961, 15, 393. (5) Charlton, J. L.; Liao, C. C.; de Mayo, P. J . Am. Chem. SOC.1971, 93, 2463. (6) Wan, P.; Yates, K. J . Org. Chem. 1983, 48, 138. (7) Cornelisse, J.; Lodder, G.;Havinga, E. Rev. Chem. Intermed. 1979, 2, 231.
0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 24, 1984 5771
Letters 25 1 0 A 4
4 x
111
1
o
p-NITROTOLUENE I N THF NITROBENZENE I N THF a-NITRUTOLUENE I N THF
100 p a 100 p a
200
NITROBENZENE I N THF
pa
I
t
/
//
I
/
J
20 &= D
z
400
450
500 550 WAVELENGTH, nm
1 650
600
10 0
-10
Figure 1. Transient absorption spectra of nitrobenzene, o-nitrotoluene, and p-nitrotoluenein THF cell pathlength. 2 mm: solute concentration. ca. 3 X M. From the mechanistic point of view, the short-lived unobserved n,a* triplet excited state has been shown to be the excited state involved in the inefficient intermolecular abstraction of nitrobenzene in isopropyl alcohol and in the photoaddition to olefins. However, the involvement and the reactivity of the n,a* excited triplet state in intramolecular processes such as the photocleavage of o-nitrophenylethylenedioxy acetals and ketalsg is less certain in view of the striking contrast in the efficiency of this processg as compared with intermolecular hydrogen abstraction by nitrobenzene. We therefore sought to detect, by picosecond transient absorption spectroscopy,the reactive excited states of nitrobenzenes which have thus far eluded detection, and to measure their kinetic behavior in an effort to obtain direct information on the nature of these excited states and their reactivity pattern. We chose, as model systems, nitrobenzene and some alkyl-substituted nitrobenzenes in T H F for initial study and used the 355-nm thirdharmonic pulse of a Nd:YAG mode-locked laser system for excitation." Pulses of 2-3 mJ, with durations of 30-40 ps fwhm, were obtained. A continuum pulse generated in 5 cm of D20 provided probe wavelengths from 400 to 650 nm. The model used for the kinetic treatment of the data was as previously described.12 Two absorption bands at ca. 440 and 625-650 nm are evident from Figure 1 which shows the transient absorption from nitrobenzene (NB), p-nitrotoluene (p-NT), and o-nitrotoluene (o-NT). The two bands for all three compounds showed a rise time comparable to the laser flash and showed similar decay patterns. o-Nitroethylbenzene (0-NEB) also displayed a similar transient (not shown) but decayed more rapidly than the other three compounds. The similar kinetic behavior and the similar spectra strongly suggest that a single transient absorbing in the two wavelength regions is produced from all three compounds. Similar spectra were obtained in acetonitrile which is evidence against the transients being associated with impurities in the THF solvent. In addition, pulse excitation of the THF and acetonitrile themselves did not give significant absorption. The transients can also be quenched by trans-piperylene. Again, both bands showed similar quenching behavior. Quenching rate were obtained for N B constants of 1 X lo9 and 0.2 X lo9 M ~~
~
(8) Dessauer, R.; Paris, J. P. In "Advances in Photochemistry", Noyes, W. A., Jr., Hammond, G. S., Pitts, J. N., Ed.; Interscience: New York, 1963; Vol. 1, p 275. (9) (a) Htbert, J.; Gravel, D. Can. J . Chem. 1974, 52, 187. (b) Gravel, D.; Htbert, J.; Thoraval, D. Can. J . Chem. 1983, 61, 400. For excellent reviews see Amit, B.; Zehavi, U.; Patchornik, A. Isr. J. Chem. 1974, Z2, 103. Pillai. W. N. R. Svnthesis 1980. 1. (10) Reichmanis, E.; Gooden; R.; Wilkins, Jr., C. W.; Schonhorn, H. J. Polym. Sci., Chem. Ed. 1983, 21, 1075. (11) Serpone, N.; Netzel, T.; Gouterman, M. J. Am. Chem. SOC.1983, 104. 145. (12) Le Sage, R.; Sala, K.L.;Yip, R. W.; Langford, C . H. Can. J. Chem. 1983, 61, 2761.
1
1 -10
0
k0
110
120
T I M E . PS
Figure 2. Rise time for the transient from nitrobenzene (ca. 3 X M) in THF. The ground, excited triplet, and excited singlet states are designated, in order of increasing energy, as level 1, 2, and 3, respectively. The rate constant for intersystem crossing is denoted by k32,and the rate of decay of the triplet by k21. The bracketed points at 120 ps are longer time points outside the range of the graph. 7 is the laser pulse width. and o-NT, respectively. Previous study on the quenching of NB by ~is-piperylene~ showed that the olefin quenching was mainly by triplet energy transfer. Thus the increased decay of the transient in the presence of trans-piperylene (ET= 59 kcal m01-l)'~ is good evidence that the transient is a triplet excited state. The quenching rate of N B is 1/20 that of the 2 X 1Olo M s-l diffusion-controlled rate constant for THF13 and is consistent with triplet energy transfer which is 1 kcal mol-' endothermic. This places the triplet energy of N B at 58 kcal mol-' and that for o-NT slightly lower. These estimates of the triplet energy of N B and o-NT, which do not appear to be unreasonable, remain to be confirmed by direct measurements. The data for N B (Figure 2) show the best fit for a calculated rise time of the transient of %a. 5 ps. The time constant for decay of o-NT (690 f 140 ps) was not significantly different from that of NB (770 f 90 ps), both monitored at 610 nm. The decay time constant for o-nitroethylbenzene (0-NEB) (450 f 35 ps) was shorter than that of either N B or o-NT. The lifetime of 800 ps for NB is in good agreement with the ca. 1-ns lifetime of the n,a* triplet deduced by Testa3 based on quantum yield measurements of cis-trans isomerization sensitized by NB and the assumption that nitrobenzene abstracts hydrogen from isopropyl alcohol at the same rate as benzophenone. As a corollary to the assignment of the observed transient to a triplet n,?r* state, the transient from o-NEB is expected to decay more rapidly than N B itself due to intramolecular abstraction of the secondary ortho H by the nitro group. This is indeed observed; the small magnitude of the increase in the decay rate constant suggests that even intramolecular abstraction of secondary hydrogens, y to the nitro group, is still in balanced competition with radiationless deactivation. If the assumption that the radiationless decay rate of o-NEB is equal to the measured decay rate of NB (1.3 X lo9 s-l) is made, where intramolecular abstraction is not a factor, the rate of reaction of o-NEB can be estimated to be ca. 0.9 X lo9 s-l, somewhat less than the radiationless decay rate. The lower measured decay rate constants for o-NT vs. o-NEB can be attributed to a rate of reaction which is 0.8 X lo9 s-l, slower than that for o-NEB, due to hydrogen abstraction from a primary vs. a secondary hydrogen. The rate of abstraction of o-NT (ca. 50.1 X lo9 s-I) is therefore expected to be quite inefficient due to other much faster competing radiationless deactivation processes. Measurements of the relative rate of disappearance of o-NT and o-NEB (1:40, re~pectively)'~ are consistent with these results, bearing in mind that interpretation of the rate of disap(13) Murov, S. L. "Handbook of Photochemistry"; Marcel Dekker: New York, 1973. (14) Giasson, R.; Gravel, D., unpublished results.
J. Phys. Chem. 1984,88, 5772-5714
5772 120
N I T R O B E N Z E N E IN 1 - P I P E R Y L -ENE
I
K l n e t I c r of t h e l o n g w a v e l e n g t h band
i
-10 -40
0
40
1
no
TIME.
120
160
zoo
‘OS
Figure 3. Kinetics of the transient from nitrobenzene (ca. 3 X lo-* M) in neat trans-piperylene. The rate constant for decay of the transient is designated by k z l . T is the laser pulse width.
pearance data has to take into account the as yet unquantified extent of ba~k-reaction.’~The present results, therefore, suggest that, in the alkyl nitrobenzenes, the rate of intramolecular hydrogen abstraction is comparable with, or less than, other radiationless processes. Intermolecular hydrogen abstraction is expected to be also inefficient, even in neat solvent, a supposition substantiated by the ca. 1% quantum yield of disappearance reported for N B in isopropyl alcoh01.~ From the value for the ratio of deactivation to hydrogen abstraction in isopropyl alcohol, kdt/kH,of 753 reported by Hurley and Testa: a rate constant for abstraction from isopropyl alcohol of 1.7 X lo6 M-I s-l can be calculated from our measured value for decay of the n,r* triplet of NB. (15)
Morrison, H.; Migdalof, B. H. J . Org. Chem. 1965, 30, 3996.
The good fit between the form of the population function of the triplet state and the measured data for the quenching of the N B triplet in neat trans-piperylene (Figure 3) showed that static quenching does not need to be invoked. This can be rationalized by the inefficiency of the T-T transfer where a large number of collisions are required per transfer and thus satisfying the condition required for the diffusion model. The rise time (55 ps) for the transient from NB, which we have assigned to a n,r* triplet excited state, is more similar to the rise times observed for 1- and 2-nitronaphthalene (10-12 ps) than to those observed for 9-nitroanthracene (50-58 ps) or 1-nitropyrene (33 ps). The variation in rise times among aromatic nitro compounds has been interpreted in terms of the electronic structure of the initial and final states involved in the intersystem crossing process.16 Our results on NB is consistent with that expected of those nitro compounds which undergo rapid intersystem crossing from an n,r* singlet to a nearby T,A* triplet state(s), which then undergo further radiationless deactivation to the lowest triplet state. For those nitro compounds which cross from either the r,r* or charge transfer (CT) excited singlet state to nearby C T or r,a* triplet states, slower intersystem crossing times are expected. For instance, in the case of 9-nitroanthracene, which has a low ionization potential, a slow buildup (50 ps) of the final triplet a,a* state has been
Acknowledgment. We thank N . Serpone, the Canadian Picosecond Laser Flash Photolysis Centre, and the Department of Chemistry, Concordia University for making this work possible. We also thank the Natural Science and Engineering Research Council and Le Ministere de 1’Education du Quibec for financial assistance to D.G. and a scholarship to R.G. Registry No. NB, 98-95-3; o-NT,88-72-2; p-NT, 99-99-0; hydrogen, 1333-74-0. (16) Ohtani, H.; Kobayashi, T.; Suzuki, K.; Nagakura, S. Chem. SOC.Jpn. 1980, 53, 43. (17) Anderson, R. W.; Hochstrasser, R. M.; Lutz, H.; Scott, G. W. Chem. Phys. Lett. 1974, 28, 153.
Phase-Dissipative Mechanisms for Laser-Induced Surface Desorption/Dissociation Processes Xi-Yi Huang, Thomas F. George,* Department of Chemistry, University of Rochester, Rochester, New York 14627
Jian-Min Yuan, and L. M. Narducci Department of Physics and Atmosphere Science, Drexel University, Philadelphia, Pennsylvania 191 04 (Received: August 13, 1984)
Vibrational excitation and relaxation for a molecule adsorbed on a surface is investigated in connection with desorption/ dissociation processes. The population equations for a laser-driven anharmonic oscillator involving both energy- and phase-relaxation mechanisms are derived with the Zwanzig projector technique and eigenfunction-expansion method due to Weidlich. Phase relaxation is seen to assist in the excitation of the resonant active mode.
Introduction Infrared laser radiation impinging onto a molecule adsorbed on a solid surface can resonantly excite the admolecular internal vibrational/rotational modes or deposit photon energy in the adbond with which the molecule is bound to the surface.’-5 It
has been shown e ~ p e r i m e n t a l l ythat ~ , ~ SF6 and CH3F molecules adsorbed on NaCl surfaces at low temperatures can be desorbed by the resonant excitation of the adsorbate internal vibration, namely, the v3 mode. The treatment of the problem of many degrees of freedom for the molecule-surface system is very com-
M. S. Slusky and T. F. George, Chem. Phys. Lett., 57,474 (1978). J. Lin and T. F. George, Sur$ Sci., 100, 381 (1980). J. Lin and T. F. George, J . Phys. Chem., 84, 2957 (1980). D. Lucas and G. E. Ewing, Chem. Phys., 58, 385 (1981). Z. W. Gortel, H. J. Kreuzer, P. Piercy, and R. Teshima, Phys. Rev.
(6) J. Heidberg, H. Stein, A. Nestmann, E. Hoefs, and I. Hussla in “Laser-Solid Interactions and Laser Processing”, AIP Conference Proceeding 50, S. D. Ferris, H. J. Leamy, and J. M. Poate, Ed., American Institute of Physics, New York, 1979, pp 49-54. (7) J. Heidberg, H. Stein, and E. Riehl, Phys. Reu. Lett., 49, 666 (1982); Surf. Sci., 126, 183 (1983).
(1) (2) (3) (4) (5) E, 27,
5066 (1983).
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0 1984 American Chemical Society
