K. B. Laughlin

Tunable far infrared laser spectroscopy of van der Waals bonds: Vibration–rotation–tunneling spectra of Ar–H2O

The first high resolution spectra of a rare gas–H2O cluster have been observed using a tunable far infrared laser to probe the vibration–rotation–tunneling levels of Ar–H2O formed in a continuous planar supersonic jet. The high sensitivity of this spectrometer facilitated extensive measurements of two perpendicular subbands which are assigned to transitions from the ground state to the upper component of a hydrogen exchange tunneling doublet (c-type) at 21 cm^−1, and to vb1 =1+ (b-type) at 25 cm^−1, the lower tunneling component of a bending vibration which is perpendicular to the tunneling coordinate. The tunneling splitting is shown to be in the range 2.5–7 cm^−1 and the lower tunneling component of the excited bending vibration lies between 39 and 43 cm^−1 above the ground state of the complex. The experimentally determined center-of-mass separation (Rc.m. =3.75 A) and harmonic stretching force constant (ks =0.0134 mdyn/A) are compared to those of related first and second row hydrides. The large amplitude motions occurring within this complex make it difficult to establish its structure.

Measurement of the perpendicular rotation‐tunneling spectrum of the water dimer by tunable far infrared laser spectroscopy in a planar supersonic jet

Fifty-six transitions from the K=1 lower-->K=2 lower tunneling–rotation band of water dimer have been measured and assigned at 22 cm^–1 by direct absorption spectroscopy in a cw planar supersonic jet expansion using a tunable far infrared laser spectrometer. Two different models were used to fit the data and several spectroscopic constants were determined for the upper and lower states. This work supports the local IAM model recently proposed by Coudert and Hougen for the hydrogen bond tunneling dynamics of the water dimer. This model includes four different tunneling motions, all of which contribute to the observed tunneling splittings. This is the most complicated hydrogen bonded system considered to be well understood at this time, at least in the lowest few K states.

Tunable far infrared laser spectrometers

Defined most conveniently in terms of modern technology, the far infrared (FIR) region of the spectrum extends from the frequencies where “standard” submillimeter techniques begin to fail ( - 10 cm - ’ = 0.3 THz) to those where lead-salt infrared diode laser technology becomes operative ( -350 cm- ’ = 10.5 THz). In this frequency range, pure rotational spectra of light molecules and torsional (hindered rotation) spectra of nonrigid molecules have been studied for many years. In terms of wavelength, this spectral region (30-1000 ,um) is centered near the peak of the room temperature (3-300 K) blackbody emission spectrum, and has accordingly been of great importance in the development of modern physics. In terms of energy, photons in this region (0.03-1.0 kcal/mole) are less energetic than essentially all known hydrogen bond (and, of course, chemical bond) strengths, but are about the same magnitude as typical van der Waals bond strengths. The extremely high resolution ( > 1 x 106) tradition

The Berkeley tunable far infrared laser spectrometers

A detailed description is presented for a tunable far infrared laser spectrometer based on frequency mixing of an optically pumped molecular gas laser with tunable microwave radiation in a Schottky point contact diode. The system has been operated on over 30 laser lines in the range 10–100 cm^–1 and exhibits a maximum absorption sensitivity near one part in 10^6. Each laser line can be tuned by ±110 GHz with first-order sidebands. Applications of this instrument are detailed in the preceding paper.

Tunable far infrared laser spectroscopy of van der Waals bonds: Extended measurements on the lowest Σ bend of ArHCl

A tunable far infrared laser system has been used to measure the vibration–rotation spectrum of the lowest Σ bending state of ArHCl near 24 cm−1 in a cw planar jet operating with a terminal jet temperature near 3 K. Over 60 transitions have been observed for both 35Cl and 37Cl isotopes with resolution of the quadrupole hyperfine structure. An improved set of molecular parameters was determined, including B, D, H, and eqQ for both upper and lower states. Very narrow linewidths (approximately 300 kHz) resulting in high resolution and sensitivity make this technique a powerful new method for the detailed investigation of intermolecular forces.

Tunable far‐infrared laser spectroscopy of hydrogen bonds: The Ka =0(u)→1(g) rotation–tunneling spectrum of the HCl dimer

The ground state K_a =0(u)→1(g) b‐type subband of the rotation–tunneling spectrum of the symmetric ^(35)Cl–^(35)Cl,^(37)Cl–^(37)Cl, and the mixed ^(35)Cl–^(37)Cl hydrogen chloride dimers have been recorded near 26.3 cm^(−1) with sub‐Doppler resolution in a continuous two‐dimensional supersonic jet with a tunable far‐infrared laser spectrometer. Quadrupole hyperfine structure from the chlorine nuclei has been resolved. From the fitted rotational constants a (H^(35)Cl)_2 center‐of‐mass separation of 3.81 A is derived for the K_a =1(g) levels, while the nuclear quadrupole coupling constants yield a vibrationally averaged angular structure for both tunneling states of approximately 20–25 deg for the hydrogen bonded proton and at least 70–75 deg for the external proton. This nearly orthogonal structure agrees well with that predicted by ab initio theoretical calculations, but the observed splittings and intensity alterations of the lines indicate that the chlorine nuclei are made equivalent by a large amplitude tunneling motion of the HCl monomers. A similar geared internal rotation tunneling motion has been found for the HF dimer, but here the effect is much greater. The ground state tunneling splittings are estimated to lie between 15–18 cm^(−1), and the selection rules observed indicate that the trans tunneling path dominates the large amplitude motion, as expected, provided the dimer remains planar. From the observed hyperfine constants, we judge the dimer and its associated tunneling motion to be planar to within 10°.

Tunable far-infrared laser spectroscopy in a planar supersonic jet: The Σ bending vibration of ArH35Cl

The lowest Σ bending vibration (02°0) of Ar-H^(35)Cl has been observed near 23.6 cm^(−1) from P(23) to R(17), in a continuous planar jet expansion using a tunable far-infrared laser. A least-squares analysis has been performed to determine the centrifugal distortion constants for the ground and excited states, which will greatly facilitate the determination of an accurate intermolecular potential surface.

Laboratory measurement of the pure rotational spectrum of vibrationally excited HCO(+) (nu2 = 1) by far-infrared laser sideband spectroscopy

Laboratory observations of the pure rotational spectrum of HCO^+ in its lowest excited bending state (v_1, v^l_2 v_3)_= (0,1^1,0) are reported. Because of their severe excitation requirements, such vibrational satellites and the high-J ground-state lines also measured here sample only hot, dense regions of matter in active molecular cloud cores and circumstellar envelopes. As the HCO^+ abundance is tied directly to the gas fractional ionization, it is probable that the vibrationally excited formyl ion transitions will provide high-contrast observations of shocked molecular material, rather than the more quiescent, radiatively heated gas surrounding stellar sources detected with the few vibrationally excited neutral species observed to date.