G. A. Bethardy

Rate and mechanism of intramolecular vibrational redistribution in the ν16 asymmetric methyl stretch band of 1‐butyne

The spectrum of the ν16 asymmetric methyl stretch vibration of 1‐butyne near 2991 cm−1 has been studied via direct absorption infrared spectroscopy at a resolution of 35 MHz. Analysis by ground state combination difference indicates that the ν16 band is extensively perturbed by dark vibrational bath states. All of the transitions appear as multiplets of about five eigenstates in a window of about 0.017 cm−1. A detailed analysis is presented for the upper state levels K’a = 0–2 and J’=0–6. A lack of J’ dependence implies anharmonic coupling is dominant and that b‐ and c‐type Coriolis interactions are not important at these low J’ values. However, the average dilution factor goes from 0.72 at the K’a = 0 to 0.46 at the K’a = 2 suggesting weak a‐type Coriolis interactions. For the K’a = 0 levels, the measured average level density of 17 states/cm−1/symmetry species is comparable to the value of 14 vibrational states/cm−1/symmetry species obtained from a symmetry specific direct count. This is an indication t...

Frequency-resolved measurement of fast intramolecular vibrational energy redistribution (IVR) in the O–H stretch of gas-phase ethanol

Abstract The frequency-resolved, molecular-beam spectrum of the O–H stretch of ethanol near 3678 cm−1 has been measured at 10 MHz resolution using an electric-resonance optothermal spectrometer (EROS). A portion of the R branch of the predominantly a-type O–H stretch for the more stable trans conformer has been rotationally assigned using a high-sensitivity infrared-microwave double-resonance technique. The spectrum shows extensive near-resonant perturbations that characterize the intramolecular vibrational energy redistribution (IVR) process. Single rovibrational transitions of the O–H stretch normal mode are found to be fractionated into more than 60 transitions due to the coupling of the O–H stretch to the background of nearby vibrational states. The overall width of the collection of molecular eigenstates coming from a single zeroth-order rovibrational state in the vibrationally excited state gives a 25 ps lifetime for energy redistribution from the O–H stretch in trans-ethanol. This lifetime is in reasonable agreement with previous time-resolved measurements of the O–H stretch lifetime of 70 ps for ethanol in CCl4. Comparison of the two measurement techniques is discussed.

Competing mechanisms for intramolecular vibrational redistribution in the ν14 asymmetric methyl stretch band of trans‐ethanol

The extensively perturbed spectrum of the asymmetric methyl stretching vibration of trans‐ethanol near 2990 cm−1 has been reinvestigated via direct absorption infrared spectroscopy at a resolution of 30 MHz. A ground state combination difference analysis of the vibrational state mixing is presented for the upper state levels Ka’ = 0–2 and J’=0–4. The analysis indicates that the rotationless 000 level is anharmonically coupled to the dark bath states. The effective number of perturbing states in each rovibrational transition increases with both J and Ka providing evidence for rotational involvement in intramolecular vibrational redistribution (IVR). The decrease of the average dilution factor from φd=0.41 at Ka’ = 0 to φd=0.09 at Ka’ = 2 and the increase of the average interaction width from Δe=0.04 cm−1 at Ka’ = 0 to Δe=0.19 cm−1 at Ka’ = 2 indicate an a‐type Coriolis component to the bright‐bath coupling. In the Ka’ = 0 series the dilution factor decreases rapidly from φd=0.92 at J’=0 to φd=0.14 at J’=3 ...

Infrared spectra of the ν2 and ν16 bands in the methyl stretch region of jet-cooled 1-butyne

Abstract Rotationally resolved infrared spectra of the ν2 and ν16 bands in the methyl stretch region of jet-cooled 1-butyne are reported at a resolution of 0.02 cm−1. Spectra at rotational temperatures of 1.8 and 4 K are assigned and fit to a rigid rotor Hamiltonian. For the a, b hybrid band ν2, we find the excited state constants (in cm−1), ν0 = 2989.4660(34), A′ = 0.8898(16), B′ = 0.1507(5), and C′ = 0.1365(2). For the c type band ν16, we find (in cm−1), ν0 = 2991.3962(46), A′ = 0.8997(8), B′ = 0.1508(7), and C′ = 0.1379(12). The quantities in parentheses are 2 SD in units of the last digits. Even at this resolution many of the lines appear as multiplets indicating substantial mixing with dark vibrational states.

Energy randomisation: how much of rotational phase space is explored and how long does it take?

The assumption that the internal energy of a molecule is randomised on a timescale that is short compared with the reaction time is at the heart of modern theories of unimolecular reaction. In applying such theories it is necessary to decide the volume of phase space in which the energy is assumed to be randomised. The question of whether the K rotational quantum number is conserved has an impact on that choice. The conceptual sequence from experimental spectra, through analysis, and interpretation in terms of K relaxation is described below.At low resolution, intramolecular vibrational energy randomisation results in the broadening of the features of IR absorption spectra. At high resolution in bound systems, such broadened features are revealed to be clumps of discrete lines, each of which is a transition to a molecular eigenstate. Since the discrete lines can be assigned by spectroscopic means, the erroneous assignment of inhomogeneous broadening to rate processes can be avoided. Each clump of eigenstates is characterised by its dilution factor, interaction width and effective level density. Examples include the IR spectra of ethanol and but-1-yne in the 3 µm region.The interpretation of molecular eigenstate spectra involves several conceptual stages: (1) identification of the bright state which would be prepared by the coherent excitation of a certain section of spectrum, (2) evaluation of the rate of energy transfer out of the bright state, (3) use of the rotational quantum number dependence of the spectra and the trends among related systems to deduce the mechanisms by which the bright state is coupled to the bath, and (4) modelling the spectra with random matrix calculations in order to determine the average coupling parameters for anharmonic coupling and x, y and z-type Coriolis interactions.Random matrix simulations provide the opportunity to address the title questions. The simulations focused particularly on rotationally mediated vibrational relaxation and were constrained to obey the rotational quantum number dependence of the Coriolis interaction. For ethanol, when the system is prepared with a specific K quantum number, one finds that K is not conserved but neither is the population completely randomised among the 2J+ 1 available K states even at long times. The time needed for the final (non-random) distribution among K-states to be achieved is typically of the order of 1 ns, even though the energy leaves the bright state an order of magnitude more quickly.

Device for high‐resolution frequency‐modulated absorption spectroscopy of jet‐cooled molecules

A device for high‐resolution frequency‐modulated absorption spectroscopy of jet‐cooled molecules is described and demonstrated. Direct absorption spectra of jet‐cooled molecules are recorded by multipassing an infrared laser beam through a free‐jet expansion with all light rays perpendicular to the nozzle axis. In sliced‐jet spectroscopy sub‐Doppler resolution is obtained by inserting a small blade upstream from the laser crossing to obstruct molecules that would otherwise absorb at the center of the Doppler profile. Here the sliced‐jet technique is extended by replacing the blade by a stainless‐steel string vibrating at a frequency of about 10 kHz. Phase sensitive detection of the frequency‐modulated absorption signal is accomplished with a lock‐in amplifier. A resolution of 12 MHz is demonstrated using the modulated sliced‐jet technique.