John C. Stephenson

Vibrationally resolved sum-frequency generation with broad-bandwidth infrared pulses.

We present a novel procedure for vibrationally resolved sum-frequency generation (SFG) in which a broad-bandwidth IR pulse is mixed with a narrow-bandwidth visible pulse. The resultant SFG spectrum is dispersed with a spectrograph and detected in parallel with a scientific-grade CCD detector, permitting rapid and high signal-to-noise ratio data acquisition over a 400-cm(-1) spectral region without scanning the IR frequency. Application to the study of a self-assembled monolayer of octadecanethiol is discussed.

Energy partitioning in the collision‐free multiphoton dissociation of molecules: Energy of ? CF2 from CF2HCl, CF2Br2, and CF2Cl2

We have developed a simple laser‐excited fluorescence method to determine the translational energy of the nascent products of infrared multiphoton dissociations, and have applied this method to the measurement of the average translational energy ET of the ? 1A1 CF2 radicals formed from the collision‐free dissociation of CF2HCl, CF2Br2, and CF2Cl2 by CO2 TEA laser pulses. The initially formed CF2 (v,J,K) is distributed in many vibrational (v) and rotational (J,K) states, and we have obtained ET (v,J,K) specifically for different values of these internal quantum numbers. ET of the CF2 is different for each parent molecule, and is independent of the intensity or wavelength of the CO2 laser for the range of values investigated. For the CF2 produced from CF2HCl, ET was the same for fragments formed with no vibrational energy and for those formed in the ν2=5 level with 3320 cm−1 of vibrational excitation, and ET was also the same for the products formed with little rotational excitation (ER?40 cm−1) and for tho...

Subpicosecond transient infrared spectroscopy of adsorbates. Vibrational dynamics of CO/Pt(111)

The vibrational dynamics of excited CO layers on Pt(111) were studied using infrared (IR) pump–probe methods. Resonant IR pulses of 0.7 ps duration strongly pumped the absorption line (ν≊2106 cm−1 ) of top‐site CO. Weak probe pulses delayed a time tD after the pump were reflected from the CO‐covered Pt(111) surface, and dispersed in a monochromator to determine the absorption spectrum of the vibrationally excited CO band, with time resolution <1 ps and monochromator resolution <1 cm−1. Transient spectra were obtained as a function of CO coverage, surface temperature, and laser fluence. Complex spectra for tD<0 show features characteristic of a perturbed free induction decay, which are expected based on multiple‐level density‐matrix models. For tD≥0, the CO/Pt absorption exhibits a shift to lower frequency and an asymmetric broadening which are strongly dependent on fluence (1.3–15 mJ/cm2 ). Spectra return to equilibrium (unexcited) values within a few picoseconds. These transient spectral shifts and the t...

Experiment and theory for CO2 laser‐induced CF2HCl decomposition rate dependence on pressure and intensity

A laser‐excited fluorescence method has been used to determine the rates at which CF2HCl molecules dissociate into CF2+HCl fragments during CO2 laser pulses of uniform fluence and known intensity. A study has been made of the dependence of the rate on CO2 laser intensity and pressure of Ar buffer gas from the collision‐free regime to atmospheric pressure. The effect of increasing Ar pressure is initially to increase the CF2HCl dissociation rate; above a moderate pressure (∼50 Torr), the rate is independent of Ar pressure up to atmospheric pressure. The data has been compared to a model, which adequately reproduces all the experimental data. The model treats the effect of collision between CF2HCl and the argon buffer gas in terms of rotational equilibration or ’’hole filling’’ in the discrete energy level region of CF2HCl. The discrete energy levels are interfaced to a quasicontinuum of vibrational–rotational states in a self‐consistent manner which incorporates a background of nonpumpable CF2HCl states as a finite heat bath interacting with the pump mode. The model is used to calculate the rate of formation of product CF2 molecules as a function of argon pressure and CO2 laser intensity. The quasicontinuum for CF2HCl is predicted to begin about four quanta above the ground state. The absorption cross section in the quasicontinuum is shown to decrease from 10−18 to 10−20 cm2 at V=15. The energy distribution in CF2HCl is predicted to be decidedly nonthermal both below and beyond threshold.

Spectroscopy and photophysics of the CF2Ã1B1-X̃1A1 system

Abstract Laser excited single vibronic level (SVL) fluorescence and SVL fluorescence excitation spectra of the low pressure vapor phase CF2 A - X transition are reported. The spectral origin is at 268.74 nm (37 197 cm−1); extensive progressions in the bending mode ν″2 = 666 ± 5 cm−1 dominate the spectra; weaker combination bands involving the symmetric stretch ν″1 = 1186 ± 15 cm−1 are observed. Fluorescence bands appear that may be assigned as originating either from the 112n or the 2n31 vibronic levels, giving ν′1 = 976 ± 24 cm−1 or ν′3 = 900 ± 20 cm−1, respectively. Measured vibronic band intensities 〈v′|v″〉2 are given for all transitions from the upper states 2n for 0 ≤ n′2 ≤ 6. The collision-free A (0,0,0) state radiative lifetime is 61 ± 3 nsec. This same lifetime is observed even for vibronic states containing 8000 cm−1 excess vibrational energy (i.e., the n′2 = 16 level). SVL fluorescence spectra and radiative lifetimes were used to calculate the transition dipole moment R e = 1.22 D for this system, and a low resolution absorption cross section σ(200; 300 K) = 6.7 × 10−19 cm2 for the A - X origin at band maximum (268.74 nm).

Picosecond Time-Resolved Adsorbate Response to Substrate Heating: Spectroscopy and Dynamics of CO/Cu(100)

The response of the molecular stretch mode of CO/Cu(100) near 2086 cm−1 (ν1) to resonant infrared, and nonresonant visible and ultraviolet pumping is measured on a picosecond time scale. Fourier transform infrared measurements establish that ν1 is anharmonically coupled to the frustrated translation near 32 cm−1 (ν4), so that transient shifts in ν1 indicate population changes in ν4. The ν1 response to visible and ultraviolet pumping is characterized by a spectral shift near zero delay time, which decays with a ≊2 ps time constant to an intermediate value, which then decays on a ≊200 ps time scale. The data agree well with a model whereby ν4 couples to both the photogenerated hot electrons and to the heated phonons. The characteristic coupling times to these two heat baths are found to both be a few picoseconds.

Population relaxation of CO(v=1) vibrations in solution phase metal carbonyl complexes

Abstract Picosecond infrared saturation-recovery experiments were performed to obtain measurements of the vibrational energy lifetimes ( T 1 ) of CO( v = 1) vibrations ( v ≈ 1920-1985 cm −1 ) of carbonyl-containing metal complexes in dilute, room-temperature solutions. For relaxation of the F 1u CO-stretching vibration of W(CO) 6 in CCl 4 , CHCl 3 , n -hexane and benzene, T 1 , was found to be T 1 = 800±200, 480±50, 140±15 and 60±6 ps, respectively, while the same mode of Cr(CO) 6 , in these solvents gave T 1 = 440±70, 295±30, 145±25 and 59±6 ps. Monocarbonyl complexes with coordinated triphenylphosphine groups (TPP) have shorter CO( v = 1 ) lifetimes. These observations are rationalized in terms of molecular structure, intramolecular bonding, solvent interaction, and energy accepting vibrational modes.

Population lifetimes of OH(v=1) and OD(v=1) stretching vibrations of alcohols and silanols in dilute solution

Picosecond infrared pump–probe experiments determined the vibrational population lifetimes (T1) of the hydroxyl fundamental stretching mode OH(v=1) in 12 alcohols (R3COH) and 8 silanols (R3SiOH) in dilute room temperature CCl4 solutions. T1 for the silanols is in the range 185<T1<292 ps, while T1 for the alcohols is much less (T1<80 ps). The deuterium‐exchanged analogs (COD and SiOD) exhibit population relaxation times similar to protonated hydroxyls. An analysis of the vibrational energy levels corresponding to modes involving the four bonds nearest the hydroxyl groups of these molecules is used to qualitatively explain the trends of the observed T1 lifetimes for these systems. Solution T1 lifetimes are also compared to those previously measured for OH(v=1) on the surface of silica and in other condensed‐phase, room temperature systems.

Vibrational energy transfer in CO from 100 to 300 °K

The laser fluorescence method, whereby CO molecules are optically pumped from the vibrational level v=0 to the v=1 state by frequency‐doubled pulses from a CO2 laser, has been used to determine vibrational energy transfer rate coefficients for CO. Rates for the V‐V exchange processes CO(0)+N2(1)→CO(1)+N2(0) and CO(1)+CO(1) →CO(0)+CO(2), and for the deactivation of CO(1) by H2 have been measured in the range 100°K ≤ T ≤ 300°K. The probability of energy transfer from N2 to CO decreases slightly as T decreases in this range, while the probability of the CO–CO V‐V process is approximately proportional to T−1. Rate coefficients were also measured at T =297°K for the deactivation of CO(1) by the polyatomic molecules CH4, C2H4, C2H6, HCOOH, CH3COOH, CH3CHO, CH3OH, C2H5OH, H2O, D2O, H2S, and C4H10.

Near‐Resonant Vibration→Vibration Energy Transfer: CO2(υ3 = 1) + M → CO2(υ1 = 1) + M* + ΔE

The laser‐excited vibrational fluorescence technique has been used to determine the rate constants for deactivation of the asymmetric stretching vibration of CO2 in collisions with CO2, CH4, C2H4, CH3F, CH3Cl, CH3Br, CH3I, BCl3, and SF6. Rates were determined as a function of temperature in the range 300–800°K. The large deactivation cross sections σ for the latter seven molecules decreased as the temperature T increased. For the latter six collision partners σ∝1 / T. This result is interpreted as a near‐resonant vibrational energy transfer process in which three vibrational quantum numbers change as the vibrational energy is shared between the collision partners.