Production and characterization of para-hydrogen gas for matrix isolation infrared spectroscopy K. Sundararajan * , K. Sankaran, N. Ramanathan, R. Gopi Chemistry Group, Indira Gandhi Centre for Atomic Research, Kalpakkam, 603102, India article info Article history: Received 9 February 2016 Received in revised form 21 March 2016 Accepted 21 March 2016 Available online 23 March 2016 Keywords: Matrix isolation Para hydrogen Ortho hydrogen Infrared Raman abstract Normal hydrogen (n-H 2 ) has 3:1 ortho/para ratio and the production of enriched para-hydrogen (p-H 2 ) from normal hydrogen is useful for many applications including matrix isolation experiments. In this paper, we describe the design, development and fabrication of the ortho-para converter that is capable of producing enriched p-H 2 . The p-H 2 thus produced was probed using infrared and Raman techniques. Using infrared measurement, the thickness and the purity of the p-H 2 matrix were determined. The purity of p-H 2 was determined to be >99%. Matrix isolation infrared spectra of trimethylphosphate (TMP) and acetylene (C 2 H 2 ) were studied in p-H 2 and n-H 2 matrices and the results were compared with the conventional inert matrices. © 2016 Elsevier B.V. All rights reserved. 1. Introduction The production of para hydrogen assumes significance (p-H 2 ) due to its wide variety of uses in several experimental techniques. To name a few, p-H 2 is used in nuclear magnetic resonance (NMR) technique to enhance the signal intensity, in matrix isolation spectroscopy as a matrix material and in the superfluidity studies [1e6]. Matrix isolation technique (MI) is a well known method of isolating the molecules of interest in a rare gas and probe them using a variety of techniques [7]. The use of solid molecular hy- drogens (H 2 ,D 2 and HD) as matrix below 4 K is well known and it is being extensively investigated [8e11]. Solid p-H 2 as a matrix host has several advantages over conventional rare gas solids [12e17]. The ground state of p-H 2 molecule is spherically symmetric with all molecules in the J ¼ 0 rotational state. As a result, the interaction between the guest molecules and host matrix (p-H 2 ) is greatly minimized and thus the spectra of the guest molecules are un- usually sharp in this host. The crystal structure of solid p-H 2 is a pure hexagonal-closed pack (hcp), which makes the optical spectra simple whereas the crystal structures of Ne and Ar matrices consist of both hcp and face-centered cubic (fcc) structures, which results in broadening of the spectra. Furthermore, p-H 2 solid has large amplitude of zero-point lattice vibration, which is characteristic of a quantum crystal. The quantum nature of the solid hydrogen is well suited for matrix isolation spectroscopy as it provides more free space for guest molecules compared to other matrices. Because of the large amplitude of zero-point lattice vibration of the solid p- H 2 , multiple trapping sites and crystal defects around the guest molecules are expected to get repaired automatically. This self repairing nature of the solid p-H 2 makes the environment around the guest molecule homogeneous. In addition, the large lattice constant of solid p-H 2 makes the interaction between the guest and the host molecules weak and as a result, the life time of the excited states of the guest molecule in solid p-H 2 becomes longer. This could be the main reason for the relatively sharper spectra of the guest molecules in solid p-H 2 matrix when compared to other solid matrices. Eventhough there are multitudinous advantages that make p-H 2 as an attractive and promising matrix material, it is a real challenge to prepare pure p-H 2 gas from n-H 2 . Normal hydrogen contains 75% o-H 2 and 25% p-H 2 . In order to prepare pure p-H 2 > 99%, the o-H 2 is to be converted to p-H 2 . There are several methods available for the preparation of pure p-H 2 [10,12,16,18e24]. Tam and Fajardo con- structed and operated a catalyst based device, which they used for pre-cooling and equilibrating the o/p composition of a hydrogen gas flow. They used rapid vapor deposition technique (flow rate of ~290 mmol/h) and could get millimeter thick transparent solid p- H 2 with a residual o-H 2 < 0.01% [16]. The enclosed cell method * Corresponding author. E-mail address: sundar@igcar.gov.in (K. Sundararajan). Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: http://www.elsevier.com/locate/molstruc http://dx.doi.org/10.1016/j.molstruc.2016.03.068 0022-2860/© 2016 Elsevier B.V. All rights reserved. Journal of Molecular Structure 1117 (2016) 181e191