R. H. Lipson

Vacuum ultraviolet laser spectroscopy. II. Spectra of Xe2 and excited state constants

Vibrationally and isotopically resolved fluorescence excitation spectra of Xe2 at 1300, 1485, and 1500 A have been obtained and analyzed. They correspond to electronic transitions from the van der Waals ground state to the three lowest excited states. Molecules of Xe2 were formed in the v‘=0 level of the ground state using a pulsed supersonic jet, and fluorescence emission was excited by a tunable, coherent, and monochromatic source of vacuum ultraviolet radiation generated by four‐wave‐sum‐mixing in Mg and Zn vapors. For each of the three band systems, unambiguous assignment of upper‐state vibrational quantum numbering was possible, resulting in the determination of accurate upper‐state spectroscopic constants for the first time. From system I (at ∼1500 A) with bands v’=36 to 43, constants for the A1u state of 129,132Xe2 were found to be T’e =63 089.9(6.9), ωe =137.48(34), ωex’e =1.1668(43), and De =4174.4(7.5) cm−1. Band system II (at ∼1485 A) with bands v’=35 to 46, gave constants for the B0+u state: T...

Enhanced Raman spectroscopy of CO adsorbed on vapor‐deposited silver

Raman spectra of CO condensed on vapor‐deposited Ag at 11 K show two signals in the CO stretching region, one originating from a monolayer next to the metal, the other from the bulk CO film above it. In addition, a low‐frequency band associated with the former was detected, as were overtones of both the low‐ and high‐frequency bands of the surface CO. Raman scattering by the former was found to be some 106 times more intense than by the latter on a per molecule basis. The state of polarization of the tow signals was investigated as a function of angle of incidence, as was the excitation profile of the signal from the CO next to the metal, using the bulk CO signal as a reference. The behavior of the latter could only be understood when one took into account coherence effects arising from the superposition of rays directly incident on or scattered by a molecule with those reflected from the metal before impinging on, or after scattering from, the molecule. These effects could give rise to apparent polarizin...

Vacuum ultraviolet laser spectroscopy. IV. Spectra of Kr2 and constants of the ground and excited states

Krypton dimers formed by supersonic jet expansion have been excited with tunable, and monochromatic VUV radiation leading to isotopically resolved fluorescence excitation spectra of three band systems at 125, 124, and 117 nm. Unambiguous upper‐state vibrational numbering was possible, resulting in evaluation of the spectroscopic constants for the three lowest excited states. From system I at ∼125 nm, with bands v’=32 to 38 and v‘=0, constants for the A 3Σ+u state of Kr2 were found to be T’e =74 303.9(8.0), ωe =244.60(43), ωex’e =2.6045(60), and De=5807.0(8.0) cm−1. Band system II at ∼124 nm, with bands v’=30 to 38 and v‘=0, led to constants for the B 1Σ+u state, T’e =75 426.8(1.7), ωe =219.50(10), ωex’e =2.2325(15), and De =5629.1(2.5) cm−1. Improved values for constants of the C 1Σ+u state were obtained from analysis of band system III at ∼117 nm (with bands v’=0 to 5, and v‘=0 to 2): T’e =85 520.60(10), ωe =43.82 (10), ωex’e =1.812(18), and De =465.3(2.0) cm−1. Vibrational constants for the ground state...

Dichromium and trichromium

Resonance and preresonance Raman spectra are obtained from chromium‐containing argon matrics. Two progressions are assigned to Cr2 and Cr3, the former with optical constants ω″e=427.5 and ω″ex″e=15.75 cm−1 and the latter with ω″e=313 and ω″ex″e=2 cm−1. When not in resonance two other lines at 123 and 226 cm−1 were observed and ascribed to the bending and asymmetric stretching vibration of Cr3, while the vibration forming the progression was taken to be the symmetric stretch. From this, Cr3 is deduced to be a molecule with C2v symmetry but with an apical angle near 60°, suggesting that it is perhaps a Jahn–Teller distorted D3h molecule.

Metal molecules, metal clusters and metal bumps

We show that metal clusters manifest a large variety of interesting spectroscopic properties ranging from such phenomena as the extremely long vibrational lifetimes of transition metal dimers, to the extreme fluxionality of form of certain metal trimers to collective electronic oscillations (surface plasmons) in larger clusters which have achieved a measure of ’’bulkness’’. The last results in the distinctive color of colloids and in the greatly enhanced Raman scattering ability of molecules adsorbed on rough silver surfaces, the metal surface irregularities sustaining similar collective conduction electron oscillations as do colloidal particles. Absorptions due to these processes in silver surface bumps have been observed directly, using a scanning, stress‐modulator ellipsometric technique.

Coherent VUV and XUV Radiation Tunable to 90 nm, and Spectra of Rare Gas Dimers

Progress in the development of coherent, tunable sources for the VUV and XUV has reached a stage where these sources will soon be found in most spectroscopic laboratories. The principal methods used in generating such radiation are frequency tripling and 4-wave sum-mixing (4-WSM) in rare gases and metal vapors [1, 2]. Recently, several novel schemes have been described which overcome the lack of transmitting window materials at λ < 104 nm. For example, rare gases as well as molecular gases have been pulsed through supersonic jets [3,4], a laminar flow of H2 has been used with a curtain of Ne buffer gas [5], and a rotating pinhole has been synchronized with pulses of the primary laser [6], all providing optical transmission during XUV generation with minimum gas flow. By these various means, it has been possible to produce XUV radiation which is tunable over limited regions, to wavelengths as short as 50 nm [7].

Spectroscopic study of Xe2 using VUV laser excitation

Fluorescence excitation of Xe2 at 149.0 nm has been observed using a pulsed supersonic jet for dimer formation and cooling, and tunable coherent VUV radiation (from 4‐wave mixing in Mg) for excitation. The resolved vibronic structure due to the many isotopic Xe2 dimers yielded unambiguous vibrational numbering (v=37 to 46) of the vibrational levels. Analysis of the spectrum gave values of Te=63,802(6) cm−1, ωe=124.5(3) cm−1, ωexe=0.934(3) cm−1, and De=4,243(6) cm−1, for the constants of the first excited state. Preliminary investigations are underway of a second band system of Xe2 at 129.5 nm using VUV radiation from 4‐wave mixing in Zn vapor, and of a Kr2 band system at 124.0 nm using Hg vapor for VUV generation.Fluorescence excitation of Xe2 at 149.0 nm has been observed using a pulsed supersonic jet for dimer formation and cooling, and tunable coherent VUV radiation (from 4‐wave mixing in Mg) for excitation. The resolved vibronic structure due to the many isotopic Xe2 dimers yielded unambiguous vibrational numbering (v=37 to 46) of the vibrational levels. Analysis of the spectrum gave values of Te=63,802(6) cm−1, ωe=124.5(3) cm−1, ωexe=0.934(3) cm−1, and De=4,243(6) cm−1, for the constants of the first excited state. Preliminary investigations are underway of a second band system of Xe2 at 129.5 nm using VUV radiation from 4‐wave mixing in Zn vapor, and of a Kr2 band system at 124.0 nm using Hg vapor for VUV generation.

Tunable, coherent sources for high‐resolution VUV and XUV spectroscopy

A review of tunable laser sources for VUV and XUV spectroscopy is presented, with emphasis on the tuning ranges and resolution achieved to date. The most useful methods are based on four‐wave frequency mixing in rare gases and in metal vapors. These sources provide tunable coherent radiation in the wavelength range of 200 to 106 nm, and in limited regions to shorter wavelengths. Typical resolving powers of 1 to 5×105 have been obtained, and values of ∼107 are within reach. These results are illustrated with examples of atomic and molecular spectra.