Knowledge Bank

The infrared spectrum of solid para hydrogen has been recorded y employing vacuum grating spectrographic technology which is ordinarily used for high resolution spectroscopy of small molecules is gas phase. The primary aim has been to measure and define the band positions and contours more accurately by simultaneously recording many calibration lines for the purpose. The initial experiment was designed to examine the nature of the $difference1$ between the measured positions and their ""calculated"" values for some of the para hydrogen transitions, amounting to as much as $5-12cm-1$. In the present investigation, attention was concentrated on the $U1(0)$ transition. In addition to the $U1(0)$ line, the zero-phonon transitions $Q1(0)+\delta a,S1(0)$ and $S0(0)+Q1(0)$ have also been observed in this study. The measured value of $5261.4cm-1$ for the $U1(0)$ was close to the ""calculated"" value of $5260cm-1$ quoted in Ref. (1). At least in this instance there does not seem to be any need to look for other probable causes for the difference between the experimental and ""calculated"" values. The integrated absorption coefficient $\alpha ¯(U1(0))$ which reflects the strength of the hexa-decapolar interaction has also been evaluated from the present experimental data. In the above experiments the crystals were grown in a brass cell fitted with sapphire windows and supported inside a liquid helium dewar. The cell was cooled by means of a copper rod attached to the bottom of the cell at one end and to a tiny copper box through which liquid helium was passed at the other end. Cells of lengths 2 mm, 5 mm, 7 mm, and 47 mm, were used in the various experiments. In order to make crystals of better optical quality the cell assembly and the cooling system have been redesigned. The various parts are being assembled and tested.