K. W. Jucks

The vibrational predissociation lifetime of the HF dimer upon exciting the ‘‘free‐H’’ stretching vibration

A computer controlled F‐center laser has been used, in conjunction with a molecular beam apparatus employing optothermal detection, to obtain an accurate measurement of the vibrational predissociation lifetime of the HF dimer following ν1 vibrational excitation. The lifetime obtained in this study is 24±2 ns. This is to be compared with a lifetime for ν2 excitation of 1.0±0.1 ns.

Structure and vibrational dynamics of the CO2 dimer from the sub‐Doppler infrared spectrum of the 2.7 μm Fermi diad

Sub‐Doppler infrared spectra of two Fermi resonance coupled bands of carbon dioxide dimer have been obtained at 3611.5 and 3713.9 cm−1 using an optothermal molecular beam color‐center laser spectrometer. The band origins for the complexes are red shifted by approximately 1 cm−1 from the corresponding ν1+ν3/2ν02+ν3 CO2 bands. The higher frequency band is perturbed while the lower frequency band appears free of extraneous perturbations as determined from a precision fit to a Watson asymmetric rotor Hamiltonian. This fit and the observed nuclear spin statistical weights reveal that the complex is planar with C2h symmetry. The C‐‐C separation and C‐‐C–O angle are determined to be 3.599(7) A and 58.2(8)°, respectively. The nearest neighbor O‐‐C distance is 3.14 A which is the same as that found in the crystal. From the centrifugal distortion analysis the weak bond stretching and symmetric bending frequencies are estimated to be 32(2) and 90(1) cm−1. No interconversion tunneling is observed.

The structure of the carbon dioxide dimer from near infrared spectroscopy

An F‐center laser–molecular beam spectrometer has been used to obtain a sub‐Doppler resolution infrared spectrum of the carbon dioxide dimer. The vibrational mode investigated in this study corresponds to the ν1+ν3 combination mode of the monomer located at 3716 cm−1. A qualitative assignment of the spectrum shows unambiguously that the equilibrium structure of the dimer is the slipped parallel, rather than the T‐shaped, geometry. The observed spectrum cannot be fit to within experimental error using conventional asymmetric rotor formalism. This may be due to a number of factors such as Fermi resonance between the upper state levels of the band and nearby levels of the dimer, such as seen in the monomer, or it could arise from tunneling effects arising from the two large amplitude motions in the dimer.

Photofragment angular distributions for HF dimer: Scalar J–J correlations in state‐to‐state photodissociation

Photofragment angular distributions have been measured for HF dimer which show resolved structure that can be assigned to individual fragment rotational channels. This data is used to establish intermolecular scalar correlations between the rotational states of the two HF fragments. The observed angular distributions are strongly dependent upon whether the ‘‘free’’ or ‘‘hydrogen bonded’’ HF stretch is initially excited. Since the infrared spectrum of the parent molecule is highly resolved, these results can be used to determine the relative state‐to‐state photodissociation cross sections. In addition, the zero point dissociation energy (D0 ) of the HF dimer is accurately determined.

The argon–hydrogen fluoride binary complex: An example of a long lived metastable system

The optothermal laser–molecular beam method has been used to measure the infrared spectrum of Ar–HF [10°0←00°0]. The results show that the vibrational predissociation lifetime of this complex is greater than the flight time of the molecules from the laser crossing region to the bolometer detector. This gives a lower limit on the lifetime of 3×10−4 s! The upper vibrational state dipole moment has also been obtained for the complex (μ1=1.495 D) by carrying out infrared stark spectroscopy. This corresponds to a 12% increase in the dipole moment upon vibrational excitation. This change can be related to a stiffening of the van der Waals bond, and hence a reduction in the amplitude of the bending motion, in the vibrationally excited state.

Near infrared spectroscopic observation of the linear and cyclic isomers of the hydrogen cyanide trimer

Sub‐Doppler resolution infrared spectra have been obtained for both the linear and cyclic conformers of the hydrogen cyanide trimer. In the case of the linear trimer, all three vibrational bands correlating with the C–H stretching fundamental of the hydrogen cyanide monomer (ν1) have been observed. The vibrational predissociation lifetime of the complex is found to be strongly mode specific. For the cyclic trimer, which has only one (doubly degenerate) infrared allowed band associated with the C–H stretch, the rotational structure is characteristic of an oblate planar symmetric top. Molecular constants are reported for both conformers. In addition, several other bands are observed in the spectrum which, although not rotationally resolved, are tentatively assigned to the tetramer.

The nitrogen–hydrogen fluoride dimer: Infrared spectroscopy and vibrational predissociation

The ν1 infrared spectrum of N2–HF has been observed at sub‐Doppler resolution using an infrared laser–molecular beam apparatus which is based on the optothermal detection method. This spectrum has been used to obtain the following molecular constants: B1=0.107 19±0.0001 cm−1 and ν0=3918.2397±0.005 cm−1. In addition, the Stark spectrum of the R1 transition was recorded in order to determine the ground (μ0=1.991±0.008 D) and excited vibrational (μ1=2.106±0.008 D) state dipole moments of the complex. Spectra recorded at the highest resolution clearly show homogeneous broadening of the individual rotational transitions giving a predissociation lifetime of 44±10 ns.

The effect of vibrational state mixing on the predissociation lifetime of ν1 excited OC–HF

Sub‐Doppler resolution infrared spectra have been obtained for the ν1 band of OC–HF. For most of the observed rovibrational transitions the linewidths are found to have a Lorentzian component of 190±10 MHz FWHM, presumably resulting from the vibrational predissociation of the complex. In several cases, however, perturbations, due to either anharmonic or Coriolis coupling between the vibrational state corresponding to the excited HF stretch and other vibrational states of the molecule, have been observed in the spectrum. Where these perturbations are present the width of the transitions vary with the relative contributions from the two states involved. This is explained in terms of a simple perturbative treatment of the coupling in conjunction with a Golden Rule treatment of the vibrational predissociation process. Stark measurements have also been performed in order to determine the dipole moment of the complex in the vibrationally excited state, namely μ1=2.545±0.008 D.

Infrared spectroscopy of the hydrogen cyanide dimer

Sub‐Doppler resolution infrared spectra have been obtained and assigned for four vibrational bands of the hydrogen cyanide dimer, namely the ν1 and ν2 C–H stretching fundamentals and two hot bands. Accurate molecular constants are reported for all of these. In the case of the free C–H stretch (ν1 ), Stark spectra have also been recorded in order to determine the dipole moments of this complex in both the ground and vibrationally excited states. The linewidths obtained for the individual rovibrational transitions indicate that the vibrational predissociation lifetime of this complex is highly mode specific.

Infrared Stark spectroscopy of the hydrogen–HF binary complex

A sub‐Doppler resolution infrared spectrum has been measured for the HF stretch of the H2–HF complex using the optothermal detection method. Infrared Stark spectra have also been obtained from which both the ground and first vibrationally excited state dipole moments have been obtained. It is clear from these measurements that the HF subunit undergoes wide amplitude bending motion even in the zero‐point bending state. Broadening of the transitions due to the finite lifetime (27 ns) of the excited vibrational state is just discernible under the highest resolution conditions.