p-H2 Matrix

Spectrocopy of an Unstable Molecule


     The use of para hydrogen (p-H2) as a matrix host has generated increasing interest because of the unique properties associated with this quantum solid. For example, effects due to an inhomogeneous environment and the possibility of having multiple trapping sites are greatly reduced in solid p-H2. Accordingly, the infrared absorption lines of guest molecules might be extremely sharp, with full widths at half maximum (~FWHM) less than 0.01 cm-1. The guest molecule seems more likely to rotate in p-H2 than in other matrices; the rotational parameters are typically 90% of those in the gaseous phase due to interaction with the hosts. The guest species might induce IR activity of the solid p-H2 host, and cooperative transitions built upon the pure vibrational transition of p-H2 and rotational levels of the guest are observed. Formation and IR absorption of CH3F. (o-H2)n clusters in a p-H2 matrix were reported when o-H2 was present at a controlled proportion. A p-H2 matrix is also an ideal medium to investigate tunneling reactions at low temperature; reactions between CH3 and H2 ,and their deuterated variants have generated interesting results.



     A nickel-plated copper plate, maintained at 3.3 K with a closed-cycle refrigerator (Janis RDK-415), served both as a cold substrate for the matrix sample and as a mirror to reflect the incident infrared (IR) beam to the detector. IR absorption spectra were recorded with a FTIR (Bomem, DA8) with a HgCdTe detector cooled at 77 K. A gaseous mixture of Sample/p-H2 (20~200 ppm) was deposited over a period of 1–3 h. Typically, 200 scans at a resolution of 0.05 cm-1 were recorded. In some experiments the matrix sample was maintained 4.5 K for 0.5-1. 0 h for annealing, but all IR measurements were performed at 3.3 K. n-H2 (99.9999%) was used after passage through a trap at 77 K before its conversion to p-H2. The efficiency of conversion is controlled by the temperature of the catalyst Fe(OH)3; the concentration of o-H2 is 100 ppm at a conversion temperature of 15 K.

Recent Projects 

  • Internal Rotation of CH3OH in Soild p-H2

     We find that a p-H2 matrix inhibits rotation of isolated CH3OH, but still allows internal rotation about the C-O bond with splittings of the E/A torsional doublet in internal-rotation-coupled vibrational modes that are qualitatively consistent with those for CH3OH in the gaseous phase. This simplified high-resolution spectrum further revealed slow conversion of nuclear spin symmetry from species E to A in the host matrix, offering potential insight into nuclear spin conversion in astrophysical sources.

  • Sping Rotating of CH3F in Solid p-H2

     When molecule can only have spinning rotation, all J levels in a K stack are quenched to the lowest one; hence only the original lowest levels (J = K) in each K stack is present, as shown in figures. If CH3F can have only spinning rotation in solid p-H2 at low temperature, we expect to observe a single line for n1n-3, but three lines for n4n-6 consisting of an intense transition of RR(0,0) and the two weak transitions of PP(1,1) and RR(1,1) on each side of the intense feature.


  1. Infrared matrix-isolation spectroscopy using pulsed deposition of p-H2,  Y.-J. Wu, X. Yang, and Y.-P. Lee, J. Chem. Phys. 120, 1168 (2004).
  2. Internal Rotation and Spin Conversion of CH3OH in Solid Para-Hydrogen, Y.-P. Lee, Y.-J. Wu, R. M. Lees, L.-H. Xu, and J. T. Hougen, Science 311, 365 (2006).
  3. Infrared spectra of C2H2 under jet-cooled and para-H2 matrix conditions, Y.-C. Lee, V. Venkatesan, Y.-P. Lee, et al., Chem. Phys. Lett. 435, 247 (2007).