Multiphoton ionization in the organic condensed phase: A three‐photon study of liquid benzene

Dissociative Photoionization of Dimethylpyridines and Trimethylpyridine at 266 nm: Dynamics of Methyl Radical Release

The 266 nm photolysis of various positional isomers of dimethylpyridines and trimethylpyridine was investigated by measuring the translational energy distribution of the methyl radical following {sp2}C-C{sp3} bond dissociation. The observed translational energy distribution is attributed to the dissociative photoionization in the cationic ground state following [1 + 1 + 1] three-photon absorption. The translational energy distribution profiles of the methyl radical were broad with the maximum translation energy in excess of 2 eV, which originates due to the dissociation of {sp2}C-C{sp3} bond ortho to the N atom in the ring. The dynamics of {sp2}C-C{sp3} bond dissociation in the cationic ground state of methylpyridines is marginally dependent on the number and position of the methyl groups; similar to xylenes, however, it is site-selective with the preferential cleavage of C-C bond in the ortho position to the pyridinic nitrogen atom, which is attributed to the relative stability of the resulting radical cation.

Time-dependent photoemission from droplets: influence of size and charge on the photophysics near the surface

Photoemission from submicrometer droplets containing a mixture of dioctyl phthalate and dioctyl sebacate was investigated by femtosecond and nanosecond photoionization. Photoelectron spectra recorded after ionization with single 4.7 eV femtosecond or nanosecond laser pulses showed marked differences between the two cases. These differences were attributed to ionization of long-lived states which only occurred within the duration of the nanosecond pulse. The tentative assignment of the long-lived states to dioctyl phthalate triplet states is discussed. A nanosecond–femtosecond pump–probe scheme using 4.7 eV (pump) and 3.1 eV (probe) pulses was used to investigate the decay dynamics of these long-lived states. The dynamics showed an accelerated decay rate at higher dioctyl phthalate concentrations. Furthermore, the dependence of the decay dynamics on droplet size and charge was investigated. The decay of the long-lived states was found to be faster in smaller droplets as well as in neutral droplets compared with both positively and negatively charged droplets. Possible mechanisms to explain these observations and the dominance of contributions from the droplets surface are discussed.

Time-resolved photoemission spectroscopy is used to study the photophysics of droplets containing dioctyl phthalate. Long-lived excited states survive longer at lower phthalate concentrations, and in larger or electrically charged droplets.

Dynamics of Methyl Radical Formation Following 266 nm Dissociative Photoionization of Xylenes and Mesitylene

The 266 nm dissociative photoionization of three xylene isomers and mesitylene leading to the formation of methyl radical was examined. The total translational energy distribution profiles [P(E_T)] for the methyl radical were almost identical for all of the three isomers of xylene and mesitylene, while a substantial difference was observed for the corresponding P(E_T) profile of the co-fragment produced by loss of one methyl group in m-xylene. This observation is attributed to the formation of the methyl radical from alternate channels induced by the probe. The P(E_T) profiles were rationalized based on the dissociation of {sp²}C-C{sp³} bond in the cationic state, wherein the {sp²}C-C{sp³} bond dissociation energy is substantially lower relative to the neutral ground state. The dissociation in the cationic state follows a resonant three-photon absorption process, resulting in a maximum translational energy of about 1.6-1.8 eV for the photofragments in the center-of-mass frame. Fitting of the P(E_T) profiles to empirical function reveals that the dynamics of {sp²}C-C{sp³} bond dissociation is insensitive to the position of substitution but marginally dependent on the number of methyl groups.

Low-Energy Electron Escape from Liquid Interfaces: Charge and Quantum Effects

The high surface sensitivity and controlled surface charge state of submicron sized droplets is exploited to study low-energy electron transport through liquid interfaces using photoelectron imaging. Already a few charges on a droplet are found to modify the photoelectron images significantly. For narrow escape barriers, the comparison with an electron scattering model reveals pronounced quantum effects in the form of above-barrier reflections at electron kinetic energies below about 1 eV. The observed susceptibility to the characteristics of the electron escape barrier might provide access to these properties for liquid interfaces, which are generally difficult to investigate.

TWO-PHOTON–INDUCED FLUORESCENCE

Nonresonant two-photon electronic spectroscopy of polyatomic molecules is reviewed for the period since 1979. Emphasis is placed on studies that expose patterns in the two-photon fluorescence (also ionization, optoacoustic) excitation spectra of aromatic hydrocarbons and the effect of vibrations and substitution, particularly within the framework of pseudoparity rules. A section is devoted to biological molecules and the emerging use of two-photon-induced fluorescence anisotropy. Relevant theoretical results are discussed, with emphasis on quantum chemical predictions of vibronic coupling and substituent effects on two-photon absorptivity and tensor properties of individual molecules. This chapter includes higher-order spectroscopy, and a limited number of three- and four-photon studies are discussed.

Laser intensity and wavelength dependence of Rose-Bengal-photosensitized inhibition of red blood cell acetylcholinesterase

The intensity and wavelength-dependence of Rose-Bengal-mediated photoinhibition of red blood cell acetylcholinesterase has been studied. Irradiation of dye-membrane suspensions with 308 nm laser excitation resulted in enzyme inhibition almost 50% greater than that obtained with 514 nm laser excitation. Sodium azide and argon purging greatly decreased the photosensitized enzyme inhibition at both wavelengths. Although Rose Bengal photosensitized enzyme inhibition more efficiently upon excitation into Sn (308 nm) than into S1 (514 nm), Stern-Volmer analysis of sodium azide quenching data gave similar quenching efficiencies at both wavelengths. Irradiation of dye-membrane suspensions with increasing intensities (Nd:YAG, 532 nm, 40 ps pulse duration) resulted in a decrease in enzyme inhibition. Saturation of the Rose Bengal fluorescence intensity and light transmission occurred with nearly the same intensity-dependence, suggesting that ground-state depletion occurs at the higher intensities. Our results demonstrate that excitation of a sensitizer into higher-lying excited singlet states can result in enhanced sensitizing efficiency. However, attempts to populate such states in Rose Bengal by sequential two-photon absorption using high intensities resulted only in ground-state depletion.