Bhavani Rajaram

Intramolecular vibrational redistribution of energy in the stimulated emission pumping spectrum of acetylene

Using a combination of low resolution dispersed A→X fluorescence spectra and high resolution stimulated emission pumping, we have spectroscopically identified the first stages of vibrational energy flow in the highly vibrationally excited acetylene prepared by A→X emission over the energy range 5 000–18 000 cm−1. A detailed study of the stimulated emission pumping (SEP) spectrum of acetylene in the EVIB=7000 cm−1 region, in which we report spectroscopic constants and rovibrational term values for 12 vibrational levels, has conclusively shown that Darling–Dennison resonance between the cis and trans degenerate bending vibrations is the first step in the redistribution of vibrational energy from the initially excited Franck–Condon bright CC stretch and trans‐bend vibrational combination levels. This allows an extension of our prior dispersed fluorescence (DF) assignments which suggested the crucial role of Darling–Dennison coupling between the cis and trans bends in IVR [J. Chem. Phys. 95, 6336 (1991)].We prove that the symmetric CH stretch vibration, previously thought to play a crucial role in the redistribution of vibrational energy, is Franck–Condon inactive. We have also shown that vibrational‐l‐resonance among the states with excitation of both degenerate bending modes, when combined with a Fermi resonance which couples CC stretch/trans/cis‐bend excited states to the antisymmetric CH stretch, determines the subsequent flow of vibrational energy after the Darling–Dennison bending resonance. These resonances all scale with vibrational excitation in nearly the simple manner expected for the lowest order anharmonic terms in the Hamiltonian, which allows the prediction of the fastest processes at high energy from a detailed study of the high resolution spectrum at lower energy. We find some interesting rules for vibrational energy flow in the short time dynamics: (i) CC stretch excitation is necessary for stretch–bend coupling; (ii) if V2‘ and V4‘ are the quantum numbers of the initially excited bright state, and vb‘ = v4‘ + v5‘ is the total bending quantum number of a state coupled to that bright state, then V4‘ ≥ vb‘ ≥ (V4‘–2V2‘); (iii) the total stretch quantum number ns‘ = (v1‘ + v2‘ + v3‘) is also conserved by the short time dynamics.These are severe and well characterized restrictions on the range of quantum numbers accessible to the initial bright state during the first stages of intramolecular vibrational redistribution of energy.Using a combination of low resolution dispersed A→X fluorescence spectra and high resolution stimulated emission pumping, we have spectroscopically identified the first stages of vibrational energy flow in the highly vibrationally excited acetylene prepared by A→X emission over the energy range 5 000–18 000 cm−1. A detailed study of the stimulated emission pumping (SEP) spectrum of acetylene in the EVIB=7000 cm−1 region, in which we report spectroscopic constants and rovibrational term values for 12 vibrational levels, has conclusively shown that Darling–Dennison resonance between the cis and trans degenerate bending vibrations is the first step in the redistribution of vibrational energy from the initially excited Franck–Condon bright CC stretch and trans‐bend vibrational combination levels. This allows an extension of our prior dispersed fluorescence (DF) assignments which suggested the crucial role of Darling–Dennison coupling between the cis and trans bends in IVR [J. Chem. Phys. 95, 6336 (1991)]....

Intramolecular vibrational relaxation and forbidden transitions in the SEP spectrum of acetylene

A 1Au→X 1Σg+ SEP spectra of acetylene near EVIB=7000 cm−1 show that Darling–Dennison resonance between the cis‐ and trans‐bending vibrations is the first step in a series of anharmonic resonances which can transfer nearly all the vibrational energy out of the Franck–Condon bright states at higher energy. In addition to allowed ‖ΔK‖≡‖K’−l‘‖=1 rotational transitions, nominally forbidden ‖ΔK‖=0,2,3 rotational transitions have also been observed due to axis‐switching and rotational‐l‐resonance. Although the range of detectable fluorescence dips is only about 30, the range of detectable SEP intensities in these spectra is probably about 500.

Rotationally resolved ultraviolet–ultraviolet double resonance study of the nonplanar Ẽ state of acetylene

In centrosymmetric molecules such as acetylene, one expects one‐photon and two‐photon spectroscopy to be mutually exclusive by the g↔u electric dipole selection rule. An ultraviolet–ultraviolet double resonance (UVUVDR) spectrum of the 74 000–78 000 cm−1 region of acetylene, recorded using the A←X transition for the first step in double resonance consists almost entirely of transitions terminating on levels of the E state identified in one‐photon vacuum ultraviolet (vuv) spectroscopy. Evidence for a nonplanar, noncentrosymmetric structure of the E state obtained by rotationally resolved fluorescence dip detected UVUVDR is presented.

Measurement of the temperature-dependent optical constants of water ice in the 15–200 µm range

The real and imaginary refractive indices of water ice in the far infrared (IR) are used in the satellite interpretation of cloud properties as well as to obtain information on ice throughout the solar system. However, few measurements of these values exist. We have measured the real and imaginary refractive indices of water ice in the far IR every 10 deg over the temperature range of 106-176 K. Ice films ranging from 0 to 140 microm thick were grown by the condensation of water vapor onto a cold silicon substrate, and the film transmission was measured from 650 to 50 cm(-1). The thickness of the ice films was determined using optical interference from a reflected He-Ne laser (lambda = 623.8 nm). The optical constants were then determined by simultaneously fitting the calculated spectra of films of varying thickness to their respective measured transmission spectra with an iterative Kramers-Kronig technique. The results are compared with previously measured data and show large discrepancies at some wavelengths while good agreement exists at others. Possible reasons for the differences are discussed. Our data clearly distinguish crystalline and amorphous ice. In addition, we note a slight shoulder in our spectra, which can be used to distinguish between cubic and hexagonal ice, although this distinction is difficult.

The Ω=1 van der Waals and Ω=0+ double well potentials of Xe 6s[3/2]01+Kr 1S0 determined from tunable vacuum ultraviolet laser spectroscopy

The laser induced fluorescence spectrum of jet‐cooled XeKr has been measured in the vicinity of the Xe 6s[3/2]01–1S0 atomic transition at 68 045.663 cm−1. The spectrum consists of two band systems, corresponding to transitions to the Ω=0+,1 electronic states from v‘=0 of the ground electronic state. By using the observed band positions and intensities, we have constructed model potentials for both excited electronic states. The Ω=0+ state has a double minimum potential [inner well, re’ = 3.09(3) A, De’ = 624(3) cm−1; outer well, re’ = 5.1(2) A, De’ = 101(1) cm−1] while the Ω=1 state potential has only a shallow van der Waals potential [re’ = 5.24(4) A, De’ = 52.2(7) cm−1]. The double minimum potential for the Ω=0+ state and the difference between the potentials for the Ω=0+ and Ω=1 states are understood in terms of the dominance of two different types of bonding interactions over different ranges of the internuclear distance. At long range, the interaction is dominated by weak dispersion and overlap repul...

Detection of the complete electric field of femtosecond four-wave mixing signals

A novel optimized heterodyne method which recovers the complete electric field of any four-wave mixing signal at its point of origin is demonstrated. A tracer pulse is sent along the signal path and characterized at the sample by frequency-resolved optical gating. Spectral interferometry is used to determine the phase difference between the tracer and a reference pulse, the absorptive change in tracer phase in the sample, and the reference-signal phase difference. Together, these measurements allow calculation of the signal phase at the sample. The phase of three pulse scattering signals from solutions of the dye IR144 in methanol determines the absolute signal emission time within 0.5 fs.

Excitation spectra of 2,5-dihydroxy-p-benzoquinone monomer and hydrates

The fluorescence excitation spectrum of the lowest allowed singlet–singlet transition of jet‐cooled 2,5‐dihydroxy‐p‐benzoquinone is reported. The transition is assigned as S3←S0, 1Bu←1Ag, π*←π and the 0–0 band origin is at 275.55 nm. Twenty vibrational levels, which include half of the Ag fundamentals, are assigned for the S3 state. The observed laser‐induced fluorescence transitions are structureless peaks with bandwidths that depend on the intensity of the excitation laser. No spectral multiplets attributable to intramolecular tunneling were observed. Deuterium isotope shifts of the 0–0 transition are +21 cm−1 per internal hydrogen bond. The 0–0 transitions of hydrate isotopomers with hydration shifts of only +11 cm−1 per water molecule are reported.