CW bleach-probe laser spectroscopy of the para-H2 Q1(0) transition in solid hydrogen crystals doped with CH3F

Abstract

In the present paper, we report a detailed experimental study of the induced Q1(0) transition of para-hydrogen (pH2) in solid hydrogen doped with CH3F. A CW quantum cascade (QC) laser (∼1039 cm−1) was used to pump transitions of CH3F arising from CH3F(ortho-H2)n clusters, while a CW distributed feedback (DFB) laser (∼4150 cm−1) probed the pH2 Q1(0) transition. The narrow spectral line width (≤10 MHz) of the DFB laser enabled the resolution of more than ten individual Q1(0) peaks with intrinsic linewidths as small as 0.0015 cm−1 (45 MHz). The relatively high power of the QC laser enabled modulation in the number distribution of CH3F(oH2)n clusters. Taking advantage of both the features, we propose a unique method of measurement named “bleach-probe spectroscopy” using the QC laser for pumping the ν3 band of CH3F and the DFB laser for probing the Q1(0) transition. By using it, we found that the induced Q1(0) transition is split into three kinds of peaks in each n-th CH3F(oH2)n cluster and the correlation between those split Q1(0) peaks and CH3F(oH2)n peaks is clarified. Furthermore, polarization spectroscopy using the DFB laser gives us the spatial direction of the induced dipole moment in pH2 around the CH3F permanent dipole moment. Combined with these results of each induced Q1(0) peaks, we suggest a primitive model of the local interactions among CH3F and surrounding (oH2)n and (pH2)12-n.