Variation of the Raman Spectrum of Solid Hydrogen With Ortho–para Ratio

Abstract

The frequencies and shapes of the vibrational Raman lines, Ql(0) and Q l ( l ) , of solid hydrogen a t 2.1 'I< have been studied for a series of ortho-para rat~os, c,/c,. In the concentration range 0.75 > c, > 0.20 and 1.0 > c, > 0.25, each Q line has a single maximum, and can be interpreted in terms of traveling vibrational excitations. For c0 < 0.20 the Q l ( l ) line splits into a band of closely spaced lines; a t these low concentrations traveling ortho excitat~ons apparently cannot be sustained, and the components of the Ql(1) line represent various nearestneighbor couplings of ortho molecules. Some observations on the Sl(0) line were also made. IKTRODUCTIONIn a n earlier investigation of the Raman spectrum of solid hydrogen (Bhatnagar et al. 1962) it was found that the frequency and half-width of the parahydrogen line, Ql(0) (v" = 0, Av = 1 ; J" = 0, AJ = 0), were not the same in normal hydrogen and in parahydrogen. The corresponding orthohydrogen line, Ql(1) (v" = 0, Av = 1 ; J" = 1 , AJ = 0), naturally did not appear in the spectrum of pure parahydrogen, but in the spectrum of normal hydrogen its frequency -\\as shifted further from the value for the low-density gas (Stoicheff 1957) than was that of Ql(0). I t was concluded that, if the changes in the frequencies and contours of these lines were followed through a series of samples of different ortho-para ratios, information concerning interactions involving ortho nlolecules could be obtained. The experiments described below were therefore undertaken. T h e Ql(0) and Q1(l) lines were studied in mixtures containing 25% t o 100% parahydrogen and the parahydrogen rotationvibrational line, Sl(0) (v" = 0 , Av = 1 ; J" = 0, AJ = 2), was studied in mixtures containing 70% to 100% parahydrogen. EXPERIMENTAL

Raman spectra of solid isotopic hydrogen mixtures

“…9,5,37 The Q 1 (J) energy dependence on c(1) reveals the coupling of the molecular vibrational states. The frequency shift for both density and c(J) from the respective gas phase line is 8,36,38,39 …”

“…The intensity of the Q 1 (J) lines are strongly dependent on c(1) in pure H 2 and D 2 . 3,8,38,39 There is an enhanced intensity of the lower energy Q 1 (1) line because, classically, the vibrating J ϭ 1 molecule drives neighboring J ϭ 0 molecules below their vibrational resonant frequency. 2 Thus, the Q 1 (1)/Q 1 (0) intensity ratio is described by…”

Theory of the Anomalous Intensities in the Vibrational Raman Spectra of Solid Hydrogen and Deuterium

“…This conclusion is reinforced by the consideration that the present model, with its regular arrangement of imBy the principle of spectroscopic stability, any increase in the total intensity of the impurity line is matched by a decrease in the total intensity of the matrix line. Thus the ratio of the intensity of the matrix line per matrix molecule to that due to an isolated molecule is We now consider the lowest paradeuterium concentration, 6=3.7%, at which measurements have been made, 11 corresponding to one impurity molecule in 27. One can arrange such a concentration of impurities in a fee matrix in a very symmetric way in which every third molecule on any given line is an impurity.…”

“…This work has grown out of study of the observed 11 unexpectedly large difference in the intensity ratio of the Qi(0) and Qi(l) Raman lines (due to transitions v=0 -» v-1, AJ= 0) for gaseous and solid hydrogen. A similar but more pronounced effect is observed 11 for deuterium.…”

“…This work has grown out of study of the observed 11 unexpectedly large difference in the intensity ratio of the Qi(0) and Qi(l) Raman lines (due to transitions v=0 -» v-1, AJ= 0) for gaseous and solid hydrogen. A similar but more pronounced effect is observed 11 for deuterium. The cross section a (J) per molecule in the state /, for the Raman scattering arising from the transitions v=0 -» v= 1, 7 -> 7, in isolated H 2 and D 2 molecules is practically independent of 7.…”