Two-photon dissociation

It shows that both the reactions are from two-photon dissociation. 

The elimination of two D atoms and two ring-opening dissociation channels were observed from the two-photon dissociation. 

Fig. 17. Photofragment ion images of C6H5CD3 in three different mass regions. Image of m/e = 76 in (b) resulted from two-photon dissociation. Image of parent molecule m/e = 95, and m/e = 96, 97 due to the 13C natural abundance are shown in (c).

Figures 9A and 9B are photofragment images of D+ following irradiation of D2 with 532-nm light. All of the features can be assigned to dissociation of different vibrational levels of D2 by nominally either one-, two, or three-photon absorption. Because two-photon absorption to the 2p

The transition to the C B- state of H 0 was achieved by a two photon absorption of KrF laser light near 248 nm (32). The OH(A-X) fluorescence excitation spectrum in the 247.9-248.5 nm range follows the rotational structure of the C B -+ X A transition. However, (i) the OH(A-X) fluorescence spectrum produced by the two photon dissociation of H 0 has a maximum population at N = 14, while single photon absorption near 124 nm generates OH fluorescence spectrum with a maximum population at N = 20 (ii) only absorption to Ka= 1 (and not Ka= 0 where K is the rotational angular momentum about the a axis) of the ClB-L state predissociates into 0H(a2%) + H probably through the B A state. Apparently, the two-photon absorption near 248 nm predominantly populates the c b state, while the single photon process populates the B A near 124 nm. 

The latter process is not competitive with the former above the thermodynamic threshold. Furthermore, as long as the available energy was kept below the thermochemical threshold for the production of vibrationally excited CN radicals, it was possible to fit the observed rotational distributions with phase space theory. The upper electronic state that is involved in the two-photon dissociation was shown to originate below 22,000 cm l and is thought to be repulsive. It could be the same state that has its absorption maximum at 270 nm. 

Resonantly enhanced two-photon dissociation of Na2 from a bound state of the. ground electronic state occurs [202] by initial excitation to an excited intermediate bound state Em,Jm, Mm). The latter is a superposition of states of the A1 1+ and b3Il electronic curves, a consequence of spin-orbit coupling. The continuum states reached in the two-photon excitation can have either a singlet or a triplet character, but, despite the multitude of electronic states involved in the computation reported J below, the predominant contributions to the products Na(3s) + Na(3p) and Na(3s) + Na(4s) are found to come from the 1 flg and 3 + electronic states, respectively. The resonant character of the two-photon excitation allows tire selection of a Single initial state from a thermal ensemble here results for vt = Ji — 0, where vt,./, denote the vibrational and rotational quantum numbers of the initial state, are stJjseussed. 

Section 8.1 introduces general rules under which this type of control is possible, in Section 8.2, we consider the two-photon dissociation of a single quantum tate of a B-A-B molecule to yield BA+B and B+AB, where B and B are Qantiomers. We demonstrate two results (1) ordinary photodissociation of the BAB molecule with linearly polarized light yields identical cross sections... 

SYMMETRY BREAKING IN TWO-PHOTON DISSOCIATION OF PURE STATES 171... 

If fset of equations used in Section 11.1 for two-photon dissociation problems can ilfqsed to address another significant problem—that of resonantly enhanced two-t mpfon association, depicted schematically in Figure 10.1c. 

Thomsen CL, Madsen D, Keiding SR, Thogersen J, Christiansen O. (1999) Two-photon dissociation and ionization of liquid water studied by femtosecond transient absorption spectroscopy./Chem Phys 110 3453-3462. 

Photodissociation of NCNO in the wavelength range 497 660—732 nm. Simultaneous one- and two-photon dissociations observed, producing rotationally cold and hot CN radicals, respectively... 

Numerical calculation of two-photon dissociation cross-sections for HJ. Cross-sections are small for low vibrational levels but increase rapidly with increasing vibrational excitation Spectroscopic study of ion photofragments resulting from the 193 nm photodissociation of H2, HD, DJ, 020", OD, ND3, and NDJ... 

An interesting technique has been developed for the determination of the kinetics of photochemical reactions using holography.The time course of the reaction is monitored by measuring the growth of an interference pattern in the sample caused by the overlap of two laser beams. The method has been used to study the two-photon dissociation of dimethyl-5> /M-tetrazineand the hydrogen-abstraction reaction benzophenone in a poly(methylmethacrylate) matrix. ... 

In some favorable cases TPE was efficient enough to form measurable amounts of photodecomposition products. Examples are the dissociation of iodoform by a ruby laser s and of water by a doubled dye laser. In the first, the reaction was followed by titrating the liberated iodine, while in the second, OH radicals were monitored by laser-induced fluorescence, using the same laser frequency for both two-photon dissociation of HjO and one-photon fluorescence excitation of OH. 

Shimizu et al report that while [2.2] paracyclophane (55) undergoes two-photon dissociation in low temperature matrices by way of the triplet state, in the gas phase, the efficient two-photon process proceeds via a hot molecule formed by internal conversion from the initially formed singlet excited state. The photocleavage of 2-nitrobenzyl ethers and ester has been widely reported and has now been evaluated as a deprotection methodology for indoles, benzimidazole, and 6-chlorouracil (Voelker et al). The mechanism of the cleavage of such compounds is considered to involve the o-quinonoid intermediate, but previously these had only been deduced from transient electronic spectra produced in flash photolysis experiments. Infrared spectral data from photochemical studies of 2-nitrobenzyl methyl ether in argon and nitrogen matrices have now been published which confirm that the intermediate does indeed have the o-quinonoid structure... 

The formation of the benzyl radical by photolysis of toluene in benzene at 4.2 and 77 K has been investigated as a function of light intensity and shown to be unimolecular and biphotonic, and to occur via the lowest excited triplet state. The gas-phase photodissociation of [2,2]paracyclophane (71) into two molecules of /7-quinodimethane and analogous dissociations of two methyl-substituted derivatives have been shown to be efficient two-photon processes.A hot molecule formed by internal conversion from the initially formed singlet electronic excited state is proposed as an intermediate, a mechanism which is completely different from that of the two-photon dissociation of (71) in low-temperature matrices, which involves a triplet state. The photolyses of [2,2]para-cylcophane and [2,2]paracyclophane-l-ene have also been studied in THF and hexane matrices at 77 K, using detection by both luminescence and absorption spectroscopy. The products from both these compounds in THF matrices included alkyl derivatives of rra/15-stilbene, and in hexane matrices alkylphenan-threnes. Cycloreversion of so-called cyclodimers of naphthalene derivatives with benzene or furan, e.g. (72), occurs efficiently upon UV irradiation. ... 

Z. Chen, M. Shapiro, P. Brumer, Incoherent interference control of two-photon dissociation, Phys. Rev. A 52 (1995) 2225. 

A. Vardi, M. Shapiro, Two-photon dissociation /ionization beyond the adiabatic approximation, J. Chem. Phys. 104 (1996) 5490 Theory of radiative recombination with strong laser pulses and the formation of ultracold molecules via stimulated photorecombination of cold atoms, A. Vardi, D. Abrashkevich, E. Frishman, M. Shapiro, J. Chem. Phys. 107 (1997) 6166. 

M. Shapiro, Theory of one- and two-photon dissociation with strong laser pulses, J. Ghem. Phys. 101 (1994) 3844 E. Frishman, M. Shapiro, Reversibility of bound-to-continuum transitions induced by a strong short laser pulse and the semiclassical uniform approximation, Phys. Rev. A. 54 (1996) 3310. 

K. Walter, R. Weinkauf, U. Boesl, E.W. Schlag, Molecular ion spectroscopy mass-selected resonant two-photon dissociation spectra of CH I+ and CDsD. J. Chem. Phys. 89, 1914 (1988)... 

Shapiro, M., Theory of one- and two-photon dissociation with strong laser pulses, J. Chem. Phys., 101, 3844-3851, 1994. 

In traps using buffer-gas cooling, photo fragmentation is an established method [15], where it is applied to polyatomic ions [15,16,85,86], In connection with sympathetic cooling of MgH+ molecular ions, two-photon dissociation was demonstrated and the branching ratio of the two possible dissociation channels. Mg - - H+ and Mg" " + H, was investigated [76]. In all of these experiments, pulsed lasers were employed. Photodissociation of HD+ is described in the following section. 

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