Resonance-enhanced photodissociation

Resonance Enhanced Photodissociation FeO States Below the Dissociation Limit... 

Unfortunately, predissociation of the excited-state limits the resolution of our photodissociation spectrum of FeO. One way to overcome this limitation is by resonance enhanced photodissociation. Molecules are electronically excited to a state that lies below the dissociation limit, and photodissociate after absorption of a second photon. Brucat and co-workers have used this technique to obtain a rotationally resolved spectrum of CoO from which they derived rotational... 

Figure 10. Vibrational spectra of the [HO—Fe—CHs] insertion intermediate in the O—H stretching region. Spectra are obtained by monitoring loss of argon from IR resonance enhanced photodissociation of the argon-tagged complexes [HO—Fe—CH3] (Ar) (n — 1,2).

Figure 11. Infrared resonance enhanced photodissociation spectrum of V (OCO)5 obtained by monitoring loss of CO2. The antisymmetric stretch of outer-shell CO2 is near 2349 cm (the value in free CO2, indicated by the dashed vertical line). The vibration shifts to 2375 cm for inner-shell CO2.

Figure 16. Experimental and calculated IR resonance enhanced photodissociation spectra of Fe" (CH4)3 and Fe" (CH4)4. Experimental spectra were obtained by monitoring loss of CH4. Calculated spectra are based on vibrational frequencies and intensities calculated at the B3LYP/ 6-311+G(d,p) level. Calculated frequencies are scaled by 0.96. The calculated spectra have been convoluted with a 10-cm full width at half-maximum (FWHM) Gaussian. The D2d geometries of Fe (CH4)4 are calculated to have very similar energies, and it appears that both isomers are observed in the experiment.

Figure 7. Resonance enhanced (1 + 1) photodissociation spectrum and rotational assignments of the Il7/2 (v = 8) (v" = 0) band of FeO. Numbers indicate / for each line the...

The general principle of detection of free radicals is based on the spectroscopy (absorption and emission) and mass spectrometry (ionization) or combination of both. An early review has summarized various techniques to detect small free radicals, particularly diatomic and triatomic species.68 Essentially, the spectroscopy of free radicals provides basic knowledge for the detection of radicals, and the spectroscopy of numerous free radicals has been well characterized (see recent reviews2-4). Two experimental techniques are most popular for spectroscopy studies and thus for detection of radicals laser-induced fluorescence (LIF) and resonance-enhanced multiphoton ionization (REMPI). In the photochemistry studies of free radicals, the intense, tunable and narrow-bandwidth lasers are essential for both the detection (via spectroscopy and photoionization) and the photodissociation of free radicals. 

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. 

A technique that has been used in laboratory studies for oxides of nitrogen and shows promise for field measurements is resonance-enhanced multiphoton ionization (REMPI) (Guizard et al., 1989 Lemire et al., 1993 Simeonsson et al., 1994). For example, Akimoto and co-workers (Lee et al., 1997) have reported a REMPI system in which a (1 + 1) two-photon absorption of light at 226 nm by NO results in ionization (vide supra). They report a detection limit of 16 ppt in their laboratory studies. Other oxides of nitrogen such as NOz and HN03 can also photodissociate in the... 

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].

Figure 8. A picosecond laser experiment to determine the photodissociation lifetime of CH3I. [Adapted from J. L. Knee, L. R. Khundar, and A. H. Zewail, J. Chem. Phys. 83, 1996 (1985).] (a) An outline of the experiment a picosecond laser pulse pumps CH3I to the repulsive X state. The 1 (or 1 ) atoms are detected as ions using a picosecond laser pulse for resonance-enhanced multiphoton ionization. (b) The I+ ion signal. Deconvolution yields a lifetime of 0.5 ps.

We have used the laser resonance-enhanced multiphoton ionization (REMPI) method to probe the final state distributions of S atoms formed in the 193-nm photodissociation of several organosulfur molecules and radicals [58-60], Using the rate-equation scheme and calibrating with the known photodissociation cross sections for the formation of S( P2,i,o, >2) in the 193-nm photodissociation of CS2, we have estimated the absolute cross sections for the photodissociation of organosulfur radicals HS [59,60] and CH3S [58,60] leading to the formation of S( P2,i,o o)-... 

The photodissociations of CH3I on LiF(001) and NaCl(001) surfaces at 248 nm have been studied by probing the CH3 fragments by angularly resolved resonantly enhanced multiphoton ionization (REMPI) and time-of-flight mass... 

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-... 

FIGURE 3, Highly schematic ( ) portrayal of the experimental arrangement [62] for the polarized laser photofragmentation of oriented CH3I molecules. The iodine atoms formed from the 1-photon photodissociation are detected by a 2+1 resonance-enhanced MPI process at an appropriate I atom resonance wavelength. [Reprinted from Ref. 

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