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Photodissociation of amino acid tryptophan in a molecular beam at wavelengths of 212.8 and 193 nm, corresponding to excitation to the second and third absorption bands, was investigated using multimass ion imaging techniques. The respective wavelengths also represent excitation to the edge of a positive circular dichroism band and the center of a negative circular dichroism band of L-tryptophan. Only one dissociation channel was observed at both photolysis wavelengths: C8NH6CH2CHNH2COOH→C8NH6CH2+CHNH2COOH. Dissociation rates were found to be 1.3×106 and 5×106 s-1 at the respective wavelengths. Comparison to theoretical calculation indicates that dissociation occurs on the ground state after internal conversion. Implication of asymmetric photolysis is discussed. [ABSTRACT FROM AUTHOR]

Photodissociation of indole at 193 and 248 nm under collision-free conditions has been studied in separate experiments using multimass ion imaging techniques. H atom elimination was found to be the only dissociation channel at both wavelengths. The photofragment translational energy distribution obtained at 193 nm contains a fast and a slow component. Fifty-four percent of indole following the 193 nm photoexcitation dissociate from electronically excited state, resulting in the fast component. The rest of 46% indole dissociate through the ground electronic state, giving rise to the slow component. A dissociation rate of 6×105 s-1, corresponding to the dissociation from the ground electronic state, was determined. Similar two-component translational energy distribution was observed at 248 nm. However, more than 80% of indole dissociate from electronically excited state after the absorption of 248 nm photons. A comparison with the potential energy surfaces from the ab initio calculation has been made. [ABSTRACT FROM AUTHOR]

Photodissociation of phenol at 248 nm was studied using multimass ion imaging techniques. Photofragment translational energy distribution of H atom elimination was measured. The results demonstrate that H atom elimination occurs on the πσ* excited state which has repulsive potential-energy functions with respect to the stretching of OH bond. It supports the recent ab initio calculation. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The photodissociation of phenol at 193 and 248 nm was studied using multimass ion-imaging techniques and step-scan time-resolved Fourier-transform spectroscopy. The major dissociation channels at 193 nm include cleavage of the OH bond, elimination of CO, and elimination of H2O. Only the former two channels are observed at 248 nm. The translational energy distribution shows that H-atom elimination occurs in both the electronically excited and ground states, but elimination of CO or H2O occurs in the electronic ground state. Rotationally resolved emission spectra of CO (1 ≤ v≤ 4) in the spectral region of 1860−2330 cm-1were detected upon photolysis at 193 nm. After a correction for rotational quenching, CO (v≤ 4) shows a nascent rotational temperature of ∼4600 K. The observed vibrational distribution of (v1)/(v2)/(v3)/(v4) 64.3/22.2/9.1/4.4 corresponds to a vibrational temperature of 3350 ± 20 K. An average rotational energy of 6.9 ± 0.7 kcal mol-1and vibrational energy of 3.8 ± 0.7 kcal mol-1are observed for the CO product. The dissociation channels, translational energy distributions of the photofragment, and vibrational and rotational energies of product CO are consistent with potential energy surfaces from quantum chemical calculations and the branching ratios from an RRKM calculation. [ABSTRACT FROM AUTHOR]

The photodissociation of p-methylphenol, p-ethylphenol, and p-(2-aminoethyl)phenol, chromophores of the amino acid tyrosine, was studied separately for each compound in a molecular beam at 248 nm using multimass ion imaging techniques. They show interesting side-chain size-dependent dissociation properties. Only one dissociation channel, that is, H atom elimination, was observed for both p-methylphenol and p-ethylphenol. The photofragment translational energy distributions and potential energy surfaces from ab initio calculation suggest that H atom elimination occurs from a repulsive excited state. On the other hand, the H atom elimination channel is quenched completely by internal conversion and/or intersystem crossing in p-(2-aminoethyl)phenol. Only C−C bond cleavage was observed from p-(2-aminoethyl)phenol. The photofragment translational energy distribution shows a slow component and a fast component. The fast component results from dissociation on an electronic excited state, but the slow component occurs only after the internal conversion to the ground electronic state. Comparison with the photodissociation of phenol and ethylbenzene is made. [ABSTRACT FROM AUTHOR]

Photodissociation of pyrimidine at 193 and 248 nm was investigated separately using vacuum ultraviolet photoionization at 118.4 and 88.6 nm and multimass ion imaging techniques. Six dissociation channels were observed at 193 nm, including C4N2H4→C4N2H3+H and five ring opening dissociation channels, C4N2H4→C3NH3+HCN, C4N2H4→2C2NH2, C4N2H4→CH3N+C3NH, C4N2H4→C4NH2+NH2, and C4N2H4→CH2N+C3NH2. Only the first four channels were observed at 248 nm. Photofragment translational energy distributions and dissociation rates indicate that dissociation occurs in the ground electronic state after internal conversion at both wavelengths. The dissociation rates were found to be >5×107 and 1×106 s-1 at 193 and 248 nm, respectively. Comparison with the potential energies from ab initio calculations have been made. [ABSTRACT FROM AUTHOR]

Photodissociation of pyridine, 2,6-d2-pyridine, and d5-pyridine at 193 and 248 nm was investigated separately using multimass ion imaging techniques. Six dissociation channels were observed at 193 nm, including C5NH5→C5NH4+H (10%) and five ring opening dissociation channels, C5NH5→C4H4+HCN, C5NH5→C3H3+C2NH2, C5NH5→C2H4+C3NH, C5NH5→C4NH2+CH3 (14%), and C5NH5→C2H2+C3NH3. Extensive H and D atom exchanges of 2,6-d2-pyridine prior to dissociation were observed. Photofragment translational energy distributions and dissociation rates indicate that dissociation occurs in the ground electronic state after internal conversion. The dissociation rate of pyridine excited by 248-nm photons was too slow to be measured, and the upper limit of the dissociation rate was estimated to be 2×103 s-1. Comparisons with potential energies obtained from ab initio calculations and dissociation rates obtained from the Rice-Ramsperger-Kassel-Marcus theory have been made. [ABSTRACT FROM AUTHOR]

The original ion imaging technique has low velocity resolution, and currently, photodissociation is mostly investigated using velocity map ion imaging. However, separating signals from the background (resulting from undissociated excited parent molecules) is difficult when velocity map ion imaging is used for the photodissociation of large molecules (number of atoms ≥ 10). In this study, we used the photodissociation of phenol at the S1 band origin as an example to demonstrate how our multimass ion imaging technique, based on modified spatial map ion imaging, can overcome this difficulty. The photofragment translational energy distribution obtained when multimass ion imaging was used differed considerably from that obtained when velocity map ion imaging and Rydberg atom tagging were used.We used conventional translational spectroscopy as a second method to further confirm the experimental results, and we conclude that data should be interpreted carefully when velocity map ion imaging or Rydberg atom tagging is used in the photodissociation of large molecules. Finally, we propose a modified velocity map ion imaging technique without the disadvantages of the current velocity map ion imaging technique. [ABSTRACT FROM AUTHOR]

Photodissociation of 2,5-dihydroxyacetophenone (DHAP), an important matrix compound in matrix-assisted laser desorption/ionization (MALDI), was studied in a molecular beam at 193 nm using multimass ion imaging techniques. Two major dissociation channels were observed, including (1) C6H3(OH)2COCH3→ OC6H3(OH)COCH3+ H and (2) C6H3(OH)2COCH3→ C6H3(OH)2+ COCH3. The minor channels include C6H3(OH)2COCH3→ C6H3(OH)2CO + CH3and/or C6H3(OH)2COCH3→ C6H3(OH)2+ CO + CH3. The photofragment translational energy distribution suggests that reaction 1 occurs at an excited state in which the potential along the O−H bond distance is repulsive. Comparison to the branching ratios from RRKM calculations suggests that reaction 2 does not occur at either the ground state or the triplet state or that if it does occur at one of these states it must not follow the RRKM model. A comparison to the photodissociation dynamics of acetophenone and phenol and its derivatives was made. [ABSTRACT FROM AUTHOR]

Matrix-assisted laser desorption ionization (MALDI) has been widely used in the mass analysis of biomolecules; however, there are a lot of debates about the ionization mechanisms. Previous studies have indicated that S1-S1 annihilation might be a key process in the generation of primary ions. This study investigates S1-S1 annihilation by examining the time-resolved fluorescence spectra of 12 matrices. No S1-S1 annihilation was observed in six of these matrices (3-hydroxy-picolinic acid, 6-aza-2-thiothymine, 2,4-dihydroxy-acetophenone, 2,6-dihydroxy-acetophenone, 2,4,6-trihydroxyacetophenone, and ferulic acid). We observed two matrix molecules reacting in an electronically excited state (S1) in five of these matrices (2,5-dihydroxybenzoic acid, a-cyano-4-hydroxycinnamic acid, 2,5-dihydroxy-acetophenone, 2,3-dihydroxybenzoic acid, and 2,6-dihydroxybenzoic acid), and S1-S1 annihilation was a possible reaction. Among these five matrices, no S1-S1 annihilation was observed for 2,3-dihydroxybenzoic acid in typical peak power region of nanosecond laser pulses in MALDI, but a very small value of reaction rate constant was observed only in the high peak power region. The excited-state lifetime of sinapinic acid was too short to determine whether the molecules reacted in an electronically excited state. No correlation was observed between the ion generation efficiency of MALDI and S1-S1 annihilation. The results indicate that the proposal of S1-S1 annihilation is unnecessary in MALDI and energy pooling model for MALDI ionization mechanism has to be modified. [ABSTRACT FROM AUTHOR]

The photodissociation of jet-cooled α-fluorotoluene and 4-fluorotoluene at 193 and 248 nm was studied using vacuum ultraviolet (vuv) photoionization/multimass ion imaging techniques as well as electron impact ionization/photofragment translational spectroscopy. Four dissociation channels were observed for α-fluorotoluene at both 193 and 248 nm, including two major channels C6H5CH2F→C6H5CH2 (or C7H7)+F and C6H5CH2F→C6H5CH (or C7H6)+HF and two minor channels C6H5CH2F→C6H5CHF+H and C6H5CH2F→C6H5+CH2F. The vuv wavelength dependence of the C7H7 fragment photoionization spectra indicates that at least part of the F atom elimination channel results from the isomerization of α-fluorotoluene to a seven-membered ring prior to dissociation. Dissociation channels of 4-fluorotoluene at 193 nm include two major channels C6H4FCH3→C6H4FCH2+H and C6H4FCH3→C6H4F+CH3 and two minor channels C6H4FCH3→C6H5CH2 (or C7H7)+F and C6H4FCH3→C6H5CH (or C7H6)+HF. The dissociation rates for α-fluorotoluene at 193 and 248 nm are 3.3×107 and 5.6×105 s-1, respectively. The dissociation rate for 4-fluorotoluene at 193 nm is 1.0×106 s-1. An ab initio calculation demonstrates that the barrier height for isomerization from α-fluorotoluene to a seven-membered ring isomer is much lower than that from 4-fluorotoluene to a seven-membered ring isomer. The experimental observed differences of dissociation rates and relative branching ratios between α-fluorotoluene and 4-fluorotoluene may be explained by the differences in the six-membered ring to seven-membered ring isomerization barrier heights, F atom elimination threshold, and HF elimination threshold between α-fluorotoluene and 4-fluorotoluene. [ABSTRACT FROM AUTHOR]