Jack M. Preses

Measurement of the absorption length and absolute quantum efficiency of TMAE and TEA from threshold to 120 nm

Several existing and planned high energy physics experiments incorporate detectors which use either TMAE (tetrakis-dimethylaminoethylene) or TEA (triethylamine) as their photosensitive agent. Understanding the operation of these devices requires knowledge of the absolute photoionization quantum efficiencies and absorption lengths of TMAE and TEA. In an experiment performed at the National Synchrotron Light source at Brookhaven National Laboratory, we have measured these parameters from 120 nm to 280 nm. The quantum efficiencies were normalized to the known photoionization yields of benzene and cis-2-butene. The results of these measurements and details of the experiment are presented in this paper.

Vacuum ultraviolet photochemistry of CH4 and isotopomers. II. Product channel fields and absorption spectra

In part I of this work the relative velocities and anisotropies of the atomic H and D fragments from methane photolysis at 10.2 eV were measured. In this paper the relative abundance of the methyl and methylene fragments are reported. A complete set of quantum yields for the different photodissociation channels of each isotopomer is obtained by combining the two sets of data. Previously it was found that H atoms are almost four times more likely than D atoms to be ejected; now it is found that hydrogen molecule photofragments are much richer in H atoms than in D. Overall, the heavier D atoms are more likely than the H atoms to remain attached to the carbon atom. An implication for astrophysics is discussed. The VUV absorption spectra of CH4 and CH3D are almost identical both at room temperature and 75 K. There is, as expected, no variation in the absorption spectrum with temperature. Evidence is given that all or almost all of the methylene is produced in the a 1A1 and not in the ground 3B1 state.

Photodissociation dynamics of cadmium and zinc dimethyl

M(CH3)2 (M=Zn,Cd) vapor was photodissociated at 193 nm into CH3 and MCH3 (A 2E) radicals and also into CH3 radicals and M atoms. The kinetic energy distribution of the CH3 was measured by a molecular beam time of flight technique. The emission spectrum of the MCH3(A 2E) was recorded in the visible region (for the first time) and it was shown that the radicals were vibrationally cold. An excitation spectrum for visible emission was determined through the VUV region down to 125 nm. Two photon production of M(n 3P01 ) states was easily observable but shown to be negligible compared to one photon processes. The methyl radicals released in this dissociation are slower but vibrationally hotter than those dissociated from CH3I.

Single‐photon induced conductivity of solutes in nonpolar solvents

Synchrotron radiation was used for determining single‐photon photoconductivity thresholds for various solutes in nonpolar solvents. For anthracene as a solute the thresholds (Eth) are 6.14, 6.07, and 5.87 eV in 2,2,4‐trimethylpentane, 2,2,4,4‐tetramethylpentane, and tetramethylsilane, respectively. The threshold decreases as the conduction band energy (V0) decreases. The polarization energy (P+) of the anthracene cation, as derived from the data for 2,2,4‐trimethylpentane, is one‐third larger in magnitude than that reported for the polarization energy of the anthracene anion in this solvent, which suggests that the cation is smaller than the anion. Thresholds were also measured for 1,2‐benzanthracene, azulene, perylene, triphenylamine, and diazabicyclo‐octane. The thresholds in solution and solvation terms are related quantitatively by the expression Eth=I.P.+V0+P+.

Studies of the 193 nm photolysis of diethyl ketone and acetone using time-resolved Fourier transform emission spectroscopy

We have observed the infrared emission from the products of the 193 nm photolysis of diethyl ketone (3‐pentanone) in comparison with acetone (2‐propanone) using time‐resolved Fourier transform spectroscopy. In the photolysis of diethyl ketone, two bands are apparent: The first, spanning the region 1950 to 2250 cm−1, is assigned to CO rovibrational transitions; the other band, spanning the region 2800 to 3400 cm−1 and not exhibiting resolved line structure, is assigned to the ethyl radical. Spectral simulations of the CO bands under conditions of minimal, but not negligible, relaxation produce a lower bound for the nascent CO rotational temperature of ∼2100 K. The CO vibrational population distribution varies slowly over the ∼80 μs time spanned by our experiment. Both the rotational and vibrational energies of CO exceed statistical partitioning in the dissociation of acetone. In comparison to the case of acetone, absolute energies in CO vibration and rotation decrease only modestly for diethyl ketone, corresponding to a dramatic increase in the excess above the statistically partitioned energies. Several simple dissociation models are compared to these results. None is fully satisfactory.

Infrared chemiluminescence from hydrogen halides produced in IR photodissociation

Abstract The dissociation of SF 6 and other molecules by pulsed CO 2 laser produces halogen that can be detected by infrared chemiluminescence resulting from their reaction with H 2 , D 2 , or C 2 H 6 . The time dependence of the chemiluminescence intensity is consistent with a direct dissociation producing F atoms with little or no excess translation energy.

The lifetime of the fluorescent excited state in solid, liquid, and vapor phase cyclohexane

The fluorescence lifetimes of cyclohexane in the solid, liquid, and vapor phase were measured using VUV synchrotron radiation from the National Synchrotron Light Source at BNL. The lifetime of liquid and solid cyclohexane changes with temperature, and the decay rate is a combination of a temperature‐dependent Arrhenius‐type process and a temperature‐independent process. The results for liquid and solid cyclohexane from 59 to −49 °C can be represented by the expression 1/τ=[5.3×108+4.2×1012 exp(−5100/1.987 T)] s−1. The latest lifetime measurements available in the literature for cyclohexane are in good agreement with our values measured between 20 and 25 °C. The lifetime of liquid cyclohexane at a given temperature is independent of the excitation energy. However, in the vapor phase the lifetime strongly depends on the excitation energy.

Infrared fluorescence from NO2 excited at 400–500 nm

NO2 has been electronically excited to the 2B2/2B1 states, using pulsed dye laser radiation at 400–500 nm. Strong mixing of the electronically excited state with the ground electronic state (2A1) leads to highly vibrationally excited NO2(2A1), from which infrared emission has been observed. The time dependence of the IR fluorescence at several wavelengths has been observed, and quenching rate constants for NO2 and other gases have been measured. In addition to IR fluorescence at wavelengths identifiable as vibrational transitions (3.0–4.0, 6.1–6.8, 7.4–8.5, and 10.0–14.0 μm), emission at wavelengths <3.0 μm has been observed and attributed to a transition with electronic character. The emission observed in these experiments has been compared with that of chemiluminescent NO2 produced in the O+NO and O3+NO reactions.

Application of external-cavity quantum cascade infrared lasers to nanosecond time-resolved infrared spectroscopy of condensed-phase samples following pulse radiolysis.

Pulse radiolysis, utilizing short pulses of high-energy electrons from accelerators, is a powerful method for rapidly generating reduced or oxidized species and other free radicals in solution. Combined with fast time-resolved spectroscopic detection (typically in the ultraviolet/visible/near-infrared), it is invaluable for monitoring the reactivity of species subjected to radiolysis on timescales ranging from picoseconds to seconds. However, it is often difficult to identify the transient intermediates definitively due to a lack of structural information in the spectral bands. Time-resolved vibrational spectroscopy offers the structural specificity necessary for mechanistic investigations but has received only limited application in pulse radiolysis experiments. For example, time-resolved infrared (TRIR) spectroscopy has only been applied to a handful of gas-phase studies, limited mainly by several technical challenges. We have exploited recent developments in commercial external-cavity quantum cascade laser (EC-QCL) technology to construct a nanosecond TRIR apparatus that has allowed, for the first time, TRIR spectra to be recorded following pulse radiolysis of condensed-phase samples. Near single-shot sensitivity of ΔOD <1 × 10−3 has been achieved, with a response time of <20 ns. Using two continuous-wave EC-QCLs, the current apparatus covers a probe region from 1890–2084 cm−1, and TRIR spectra are acquired on a point-by-point basis by recording transient absorption decay traces at specific IR wavelengths and combining these to generate spectral time slices. The utility of the apparatus has been demonstrated by monitoring the formation and decay of the one-electron reduced form of the CO2 reduction catalyst, [Rei(bpy)(CO)3(CH3CN)]+, in acetonitrile with nanosecond time resolution following pulse radiolysis. Characteristic red-shifting of the ν(CO) IR bands confirmed that one-electron reduction of the complex took place. The availability of TRIR detection with high sensitivity opens up a wide range of mechanistic pulse radiolysis investigations that were previously difficult or impossible to perform with transient UV/visible detection.

Fluorescence lifetime and quenching rate measurements of the ultraviolet and visible emission bands of the CF3 radical

Abstract Vacuum UV synchrotron radiation was used to produce electronically excited CF 3 radicals by photolysis of CF 3 Cl, CF 3 Br, and CF 3 I. The collision-free fluorescence lifetime of both the UV and visible emission bands of CF 3 was measured to be 14–17 ns using a time-resolved photon-counting detection system. The quenching rates of the UV and visible band emissions by CF 3 precursor molecules are (1–6) × 10 7 s −1 Torr −1 .