Photodissociation vibrationally mediated

Crim F F 1993 Vibrationally mediated photodissociation exploring excited state surfaces and controlling decomposition pathways Ann. Rev. Rhys. Chem. 44 397-428... 

Vibrationally mediated photodissociation (VMP) can be used to measure the vibrational spectra of small ions, such as V (OCO). Vibrationally mediated photodissociation is a double resonance technique in which a molecule first absorbs an IR photon. Vibrationally excited molecules are then selectively photodissociated following absorption of a second photon in the UV or visible [114—120]. With neutral molecules, VMP experiments are usually used to measure the spectroscopy of regions of the excited-state potential energy surface that are not Franck-Condon accessible from the ground state and to see how different vibrations affect the photodissociation dynamics. In order for VMP to work, there must be some wavelength at which vibrationally excited molecules have an electronic transition and photodissociate, while vibrationally unexcited molecules do not. In practice, this means that the ion has to have a... 

The V (OCO) ion has a structured electronic photodissociation spectrum, which allows us to measure its vibrational spectrum using vibrationally mediated photodissociation (VMP). This technique requires that the absorption spectrum (or, in our case, the photodissociation spectrum) of vibrationally excited molecules differ from that of vibrationally unexcited molecules. The photodissociation spectrum of V (OCO) has an extended progression in the V OCO stretch, indicating that the ground and excited electronic states have different equilibrium V "—OCO bond lengths. Thus, the OCO antisymmetric stretch frequency Vj should be different in the two states, and the... 

Figure 14. Mode selectivity in photodissociation of V (OCO). The ratio of the reactive (VO + CO) to nonreactive (V + CO2) product is measured at the peaks of the vibronic bands labeled in Fig. 13. The data below 16,600 cm is from bands accessed by one-photon excitation data at higher energy was obtained by vibrationally mediated photodissociation exciting the OCO antisymmetric stretch.

Experimentally one can investigate resonances by various spectroscopic schemes, as indicated in Fig. 1 by direct overtone pumping [11] from the ground vibrational state, by vibrationally mediated photodissociation [12] using an excited vibrational level as an intermediate, or by stimulated emission pumping (SEP) [13-15] from an excited electronic state. In all cases it is possible to scan over a resonance and thereby determine its position j4s aHd its width hkU). A schematic illustration of an absorption or emission spectrum is depicted on the left-hand side of Fig. 1 all of the more or less sharp structures at energies above threshold are resonances. Figure 2 shows an overview SEP spectrum measured for DCO [16]. It consists of... 

Photodissociation dynamics [89,90] is one of the most active fields of current research into chemical physics. As well as the scalar attributes of product state distributions, vector correlations between the dissociating parent molecule and its photofragments are now being explored [91-93]. The majority of studies have used one or more visible or ultraviolet photons to excite the molecule to a dissociative electronically excited state, and following dissociation the vibrational, rotational, translational, and fine-structure distributions of the fragments have been measured using a variety of pump-probe laser-based detection techniques (for recent examples see references 94-100). Vibrationally mediated photodissociation, in which one photon... 

Brouard, M., Martinez, M.T., and O Mahony, J. (1990). Fragment pair correlations in the vibrationally mediated photodissociation of H2O2 Rotation-vibration coupling in the third OH stretching overtone, Mol. Phys. 71, 1021-1041. 

Vander Wal, R.L. and Crim, F.F. (1989). Controlling the pathways in molecular decomposition The vibrationally mediated photodissociation of water, J. Phys. Chem. 93, 5331-5333. 

Direct excitation to the continuum usually (but not always, vide infra) results in rupture of the weakest bond. In order for the experimenter to have control over which bond is broken, it is helpful first to excite motion along the bond of interest. This process, known as vibrationally mediated photodissociation, preselects the desired degree of freedom before the reaction takes place. This method is illustrated in Fig. 1, where a low energy photon excites... 

FIGURE 2 Vibrationally mediated photodissociation of water. (Provided by the courtesy of Fleming Crim.)... 

Brown, A. and Baliut-Kurti, G.G. (2000) Spin-orbit branching in the photodissociation of HE and DF II. A time-dependent wave packet study of vibrationally mediated photodissociation, J. Chern. Phys. 113, 1879-1884. 

An interesting application of overtone excitation is vibrationally mediated photodissociation, in which a molecule excited to high vibrational levels is photodissociated to a repulsive state (Likar et al., 1988 Crim, 1990). Figure 5.9 shows schematically the process in question. The excited repulsive surface can be reached by either one... 

Figure 5.9 Schematic potential energy surfaces for two-color vibrationally mediated photodissociation spectroscopy. The location of the initial excitation on the upper repulsive surface is varied by the choice of the intermediate vibrational energy. Taken with permission from Likar et al. (1988).

F.F. Grim, Vibrationally mediated photodissociation—exploring excited state surfaces and controlling decomposition pathways, Annu. Rev. Phys. Chem. 44 (1993) 397. 

Selective bond breaking has been demonstrated with HOD by first exciting the fourth overtone (local mode) of the OH bond and then photodissociating the molecule via the A X transition. The A <— X transition is red shifted (hot-band absorption) into the 240-270 nm region and the dissociation of the OH bond, relative to the OD bond, is enhanced by a factor of 15. This type of process is referred to as vibrationally mediated photodissociation and can be a very effective approach, provided the initial vibrational excitation remains localized in one chemical bond for a sufficient length of time to allow further excitation and dissociation. In the case of HOD it is clear that randomization of the vibrational energy is slower than the photodissociation step, and this further emphasizes the direct and impulsive nature of dissociation on the A Bi-state PES. 

One of the more intriguing questions concerning the tunneling dynamics in ammonia photodissociation is the impact of vibrational excitation prior to the electronic transition. Grim and coworkers have examined the action spectra of such vibrationally mediated photodissociation processes in NH3 and found drastically... 

Hause ML, Yoon YH, Crim FF (2006) Vibrationally mediated photodissociation of ammonia the influence of N-H stretching vibrations on passage through conical intersections. J Chem Phys 125 174309... 

Vibrationally mediated photodissociation of molecules via excited electronic states... 

Probably the first suggestion for utilizing the properties of laser light (the high intensity and short duration of radiation pulses) was (Letokhov 1969) to use the vibrationally mediated photodissociation of molecules via an excited repulsive electronic state with noncoherent isotope-selective saturation of the vibrational transition (Fig. 11.2). The isotope-selective two-step photodissociation of molecules consists of pulsed isotope-selective excitation of a vibrational state in the molecules by IR laser radiation and subsequent pulsed photodissociation of the vibrationally excited molecules via an excited electronic state by a UV pulse (Fig. 11.2(a)) before the isotope selectivity of the excitation is lost in collisions. Selective two-step photodissociation of molecules is possible if their excitation is accompanied by a shift of their continuous-wave electronic photoabsorption band. In that case, the molecules of the desired isotopic composition, selectively excited by a laser pulse of frequency uji, can be photodissociated by a second laser pulse of frequency uj2 selected to fall within the region of the shift where the ratio between the absorption coefficients of the excited and unexcited molecules is a maximum (Fig. 11.2(b)). 

Access to the transition state region vibrationally mediated photodissociation... 

A simplified, one-dimensional, view of vibrationally mediated photodissociation is shown in Figure 7.15. The sequential use of an IR photon and a UV photon accesses the repulsive upper state more efficiently than a single UV photon of their combined energy. The vibrational excitation allows a bond stretch on the... 

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