Michael P. Casassa

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...

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 deactivation of surface OH chemisorbed on SiO2: Solvent effects

Picosecond infrared transmission spectroscopy was used to directly measure the vibrational energy relaxation time T1 of hydroxyl groups chemisorbed on the surface of colloidal silica (SiO2). T1 was obtained for OH(νstretch=1) in the strongly bound ‘‘isolated sites’’ of fumed silica particles in vacuum and dispersed in several liquids at T=293 K. At the SiO2/vacuum interface, T1=204±20 ps. When the SiO2 particles are surrounded by solvents, the relaxation time of the surface OH(v=1) groups decreases: for the liquids CCl4, CF2Br2, CH2Cl2, and C6H6, T1(ps)=159±16, 140±30, 102±20, and 87±30, respectively. T1 does not depend on the size of the SiO2 particles for the range 70 A≤ diameter ≤150 A, or on the surface OH coverage up to an average density of 4 OH/100 A2. Significant amounts of physisorbed water (5 H2O/100 A2) decreased T1 for the isolated OH(v=1) to T1=56±10 ps. For comparison to the surface hydroxyls, the vibrational deactivation time for OH(v=1) groups in the bulk of fused silica (OH/SiO2≊130 ppm b...

Time‐ and state‐resolved measurements of nitric oxide dimer infrared photodissociation

Picosecond and nanosecond lasers and pulsed molecular beam techniques have been used to measure the infrared photodissociation spectra, the product state distributions, and the predissociation lifetimes of vibrationally excited nitric oxide dimer (NO)2 . Results for the ν1 (v=1) symmetric NO stretching mode and the ν4 (v=1) antisymmetric NO stretching mode are presented. Predissociation lifetimes are determined by time‐resolved laser induced fluorescence probing of the NO monomer product appearance rate. A dramatic mode dependence of the predissociation lifetimes is observed with the higher energy ν1 mode decaying in approximately 1 ns, and the lower energy ν4 mode decaying in approximately 40 ps. The mode dependence is independent of which product state is probed. The product state distributions show that 75% to 80% of the available energy is channeled into relative translational energy of the fragments for both modes. Rotational state distributions are Boltzmann‐like with temperatures ranging from 71 to...

Time‐resolved measurements of OH(v=1) vibrational relaxation on SiO2 surfaces: Isotope and temperature dependence

Picosecond infrared spectroscopy was used to measure the vibrational energy relaxation time T1 of OH(v=1) and OD(v=1) groups chemisorbed on silica surfaces over the temperature range 100≤T≤800 K. The observed T1 times and their temperature dependencies are discussed in terms of a multiphonon relaxation mechanism. Limiting low temperature lifetimes are T1=220±20 ps (1σ) for OH(v=1) and T1=149±10 ps for OD(v=1).

Temperature dependence of the vibrational population lifetime of OH(ν = 1) in fused silica

Abstract An infrared picosecond transient bleaching technique was used to measure vibrational lifetimes ( T 1 ) of hydroxyl groups in fused silica over the temperature range 100–1450 K. T 1 decreases from 109 to 15 ps in this range. The T 1 temperature dependence is compared to non-radiative relaxation theory for the decay of the OH(ν = 1) quantum by a multiphonon mechanism.

Overtone‐excited HN3(X̃ 1A’): Anharmonic resonance, homogeneous linewidths, and dissociation rates

High‐resolution spectra have been obtained for the predissociative N–H stretching overtone levels 5ν1 (15 120 cm−1) and 6ν1 (17 670 cm−1) of HN3, cooled in a free‐jet expansion. The spectral bandwidth (Doppler limited at 0.007 cm−1 full width at half‐maximum (FWHM) is sufficiently narrow to reveal the homogeneous linewidths of individual rovibrational transitions in the 6ν1 band, for which we previously measured the dissociation lifetime in time‐domain experiments. Two distinct manifestations of vibrational coupling characterize the spectra: (a) anharmonic mixing of the N–H stretch with other vibrational motions to give a complex spectrum of vibrational eigenstates and (b) homogeneous widths of the resultant states determined by the dissociation lifetime. The results are discussed with reference to previous studies of overtone spectroscopy and intramolecular mixing. Time‐domain measurements of dissociation rates are reported for four vibrational levels with zero‐order labels 5ν1 +νx. Over the range 15 100...

Unimolecular dynamics following vibrational overtone excitation of HN3 v1=5 and v1=6: HN3(X̃;v,J,K)→HN(X 3Σ−;v,J,Ω)+N2 (X 1Σ+g)

Excitation of the NH‐stretch overtone transitions of HN3 to v1=5 and 6 resulted in predissociation to HN(X) and N2(X) with lifetimes of 80+60−30 and ≤3 ns, respectively. Following excitation of either overtone, the HN fragments were formed predominantly in the symmetric F1, F3 spin–rotation states, with less than 4% population in the antisymmetric F2 levels. Fragment Doppler profiles confirmed that most of the available energy (>96%) went into translational motion.

Dissociation lifetimes and level mixing in overtone‐excited HN3(X̃ 1A’)

Vibrational overtone photodissociation is used to examine the spectroscopy and vibrational predissociation lifetimes of HN3 in its ground electronic state. Direct overtone pumping of the N–H stretching levels 5νNH and 6νNH prepares molecules in selected states (v,J,K) near 15 100 and 17 700 cm−1 of vibrational energy; spin‐forbidden NH(X 3 Σ−) dissociation fragments are detected by laser‐induced fluorescence. Photodissociation spectra of beam‐cooled HN3 display mixing of individual rotational levels of the nνNH vibrations with several background states, with derived coupling matrix elements in the range 0.01–0.1 cm−1. Vibrational predissociation lifetimes of mixed components of 5νNH are state specific, with variations of a factor of 2 for only 0.1 cm−1 energy differences. Average lifetimes for low J, K are 210 ns for 5νNH and 0.95 ns for 6νNH. The ratio of decay rates for the two overtone levels, k(6νNH)/k(5νNH)=220, is much greater than predicted by statistical theory, which gives a ratio of 4.

Nascent product states in the photoinitiated reaction of O3 and H2O

The rotational, vibrational and fine‐structure state distributions for the reaction 16O(1D)+ H218O →16OH +18OH, triggered by 266 nm photolysis of O3, have been measured under conditions where less than 1% of the nascent fragments experienced collisions prior to detection. The distributions are qualitatively different than those reported earlier for 266 nm photolysis, which were evidently affected by collisions. The rotational and vibrational state distributions are similar to recent 248 nm photolysis experiments, but with differences attributed to collisional and/or energetic effects in those experiments. The ‘‘new’’ 16OH is formed with vibrational populations in the ratio 0.39(v=0):0.29(v=1):0.3(v≥2). Gaussian rotational energy distributions peaked near N=12 give average rotational energies of 〈Erot〉 = 3440 and 2780 cm−1 for 16OH v=0 and v=1, respectively. The ‘‘old’’ 18OH is much colder with vibrational populations 0.94(v=0):0.06(v=1) and a 18OH v=0 Gaussian rotational energy distribution characterized ...