Resonance enhanced multiphoton dissociation

Figure ID. If a tunable laser is used for secondary excitation of molecular ions, resonance dissociation spectroscopy of molecular cations may be performed. Here excited cationic levels are subject to laser spectroscopy. They serve as intermediate states for the process of resonance enhanced multiphoton dissociation. This is quite similar to resonance ionization spectroscopy of neutrals. The difference is that a dissociation instead of an ionization continuum is finally reached by multiphoton excitation. The advantage of this technique is that it is independent of high ion numbers (as necessary for absorption spectroscopy), fluorescence [necessary for laser-induced fluorescence (LIF)] or predissociation and therefore is fairly general. In addition, mass selectivity is intrinsic and one may benefit from state selective ion formation if resonance multiphoton ionization is used as an ion source.

The ionization of ammonia clusters (i.e. multiphoton ionization,33,35,43,70,71 single photon ionization,72-74 electron impact ionization,75 etc.) mainly leads to formation of protonated clusters. For some years there has been a debate about the mechanism of formation of protonated clusters under resonance-enhanced multiphoton ionization conditions, especially regarding the possible alternative sequences of absorption, dissociation, and ionization. Two alternative mechanisms63,64,76,77 have been proposed absorption-ionization-dissociation (AID) and absorption-dissociation-ionization (ADI) mechanisms see Figure 5. 

Nitrosobenzene was studied by NMR and UV absorption spectra at low temperature146. Nitrosobenzene crystallizes as its dimer in the cis- and fraws-azodioxy forms, but in dilute solution at room temperature it exists only in the monomeric form. At low temperature (—60 °C), the dilute solutions of the dimers could be obtained because the thermal equilibrium favours the dimer. The only photochemistry observed at < — 60 °C is a very efficient photodissociation of dimer to monomer, that takes place with a quantum yield close to unity even at —170 °C. The rotational state distribution of NO produced by dissociation of nitrosobenzene at 225-nm excitation was studied by resonance-enhanced multiphoton ionization. The possible coupling between the parent bending vibration and the fragment rotation was explored. 

Photolysis of the dimer, reaction (44), proceeds primarily via generation of Cl + ClOO (Cox and Hayrnan, 1988 Molina et al., 1990). For example, Molina et al. (1990) reported the quantum yield for this channel at 308 nm to be unity, with an uncertainty of 30%. Okumura and co-workers (Moore et al., 1999) and Schindler and co-workers (Schmidt et al., 1998) have reported that the quantum yield is less than 1.0. For example, Schmidt et al. (1998) used resonance-enhanced multiphoton ionization (REMPI) with time-of-flight (TOF) mass spectrometry to follow the production of oxygen and chlorine atoms as well as CIO in vibrational levels up to v" = 5 in the photolysis of the dimer. At a photolysis wavelength of 250 nm, the quantum yield for chlorine atom production was measured to be 0.65 + 0.15, but CIO was not observed. Assuming that all of the excited dimer dissociates, this suggests that the production of CIO in vibrational... 

Figure 2.5 Schematics of the concepts for (a) ionization-loss stimulated Raman spectroscopy (ILSRS), where resonant two photon ionization of the molecular parent, follows the depletion of ground state reactant species as a result of SRS and (b) VMP, where preexcited molecules are dissociated and the ensuing H photofragments are probed by (2 -I-1) resonantly enhanced multiphoton ionization. Reproduced with permission from Ref. [86]. Copyright (2009) AIP Publishing LLC.

Figure 7. Schematic energy level diagram showing the principle of the ionization method for detecting electron transfer in gas-phase adducts. Naphthalene cation (the hole donor) is formed by resonance-enhanced two-photon ionization of the neutral. A hole acceptor, whose ionization potential is lower than that of naphthalene, is not ionized, since its S level is not resonant with the UV photons used (vi). The vibrational levels of the ionic form of the acceptor are resonant with the naphthalene cation, and accept the hole easily. Detection is by photodissociation, using photons of different frequency (V2) that dissociate the naphthalene cation in a resonance-enhanced multiphoton absorption process. Charge transfer is detected by the diminution of the product ion signal in the presence of a suitable acceptor. Adapted from Ref. [32].

More detailed investigations [118] included a deeper look at the SO rovibrational spectrum and a resonance enhanced multiphoton ionization (REMPI) analysis of the methyl radicals. These resulted in total rovibrational energies for the fragments. The translational energy component was not measured. The quantum yield for formation of SO in its ground electronic state ( Z) was unity within experimental error. Evidence for two types of methyl radicals, as might be expected for stepwise decomposition, was not found, so the authors suggested that the three-body dissociation pathway was dominant [118]. 

The competition between dissociation and fluorescence produces such a dramatic effect in this case because the radiative lifetime is very long (- 23 ysec). The C state of H2O furnishes a very different example. The photodissociation dynamics of this state have been probed using multiphoton excitation, with detection either of ions or of fluorescence. The 3+1 resonance enhanced multiphoton ionisation spectra of the C states of H2O and D2O are dominated by levels with low K, and particularly (Ashfold, Bayley and Dixon 1984). The two-... 

The product state distribution can be measured directly, for example by laser induced fluorescence (LIF) or resonance enhanced multiphoton ionisation (REMPI). Both of these techniques yield the quantum specific density of AB molecules that are created in the dissociation process. However, these methods can be applied only to a limited number of molecules. Whereas LIF is essentially restricted to a few diatomic molecules, REMPI allows in a few favoured cases also the state selective detection of larger molecules. 

The characterization of the X and a state levels of homonuelear and heteronuclear alkali dimer molecules formed by decay of the upper levels formed by photoassociation is discussed next. Resonance-enhanced multiphoton ionization is shown to be a powerful technique for establishing the population of vibrational levels formed in the X and a states near the dissociation limit. A new ion depletion technique for observing rotational and hyperfine structure as well for these levels near dissoeiation is also discussed. To reach lower levels, especially in the ground X state, it is useful to select specific photoassociation approaches such as double minimum excited-state potentials, resonant coupling of two (or more) excited state potentials, and stimulated Raman transfer from levels near dissociation to low levels (e.g., the collisionally stable V = 0,J = 0 level). 

Figure 1 Illustration of (A) sequential and (B) simultaneous two-photon excitation from state A to state B. Also shown in (B) are three possible fates of the excited state B fluorescence, dissociation and further photon absorption that ionizes the molecule. This latter process it termed 2+1 resonance-enhanced multiphoton ionization (REMPI).

Figure 1.3 HD rotational and vibrational state distributions measured for the H + D2 reaction at a collision energy of 1.3 eV. The energy is determined by the recoil energy of the H atom in the photodissociation of HI at a wavelength where it dissociates primarily to ground state I atoms. The experimental results shown [adapted from D. P. Gerrlty and J. J. Valentini, J. Chem. Phys. 81, 1298, (1984) and Valentin and Phillips (1989)] used CARS spectroscopy to determine the state of HD. E. E. Marinero, C. T. Rettner, and R. N. Zare, J. Chem. Phys. 80,4142 (1984) used resonance enhanced multiphoton ionization, REMPI, for this purpose. The figure also shows curves. Those on the left are the so-called linear surprisal representation, see Section 6.4. The plot on the right shows the same experimental data on a logarithmic scale. The curves [adapted from N. C. Blais and D. G. Truhlar, J. Chem. Phys. 83, 2201 (1985)] are a dynamical computation by the method of classical trajectories. Section 5.2.

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