Parahydrogen Crystal Research Articles

Tunable-narrow-linewidth continuous-wave mid-infrared light generation by difference-frequency mixing

The generation of tunable narrow-band mid-infrared light by difference-frequency mixing of two kinds of continuous-wave ring-laser radiation in potassium titanyl phosphate is described. The system produces infrared powers of ∼20 µW of difference-frequency light continuously tunable from 1.05 to 2.8 µm (3500–9500 cm-1). Continuous scans over a range of 100 cm-1 with a spectral precision better than 10 MHz (0.0003 cm-1) are achieved. The high-resolution characteristics of the laser system are evaluated by use of a portion of Q2(0) overtone vibration–rotation spectrum of a solid parahydrogen crystal.

Direct measurement of the crystal field splitting of isolated J=1 impurities in solid parahydrogen

The crystal field splitting of single J=1 impurity orthohydrogen molecules in an otherwise pure J=0 parahydrogen crystal has been studied experimentally and theoretically over many years. We have measured this quantity directly from the fine structure of the Q3 (0) (v = 3← 0, J = 0← 0) overtone transition induced by J=1 hydrogen, in the near infrared using a Ti:sapphire laser. From our observed eight pairs of split spectral lines, we have determined the value Δ/k = [E(M=±1) − E(M=0)]/k = 0.0102 ± 0.0003 K including its sign. A straightforward numerical calculation based on ab initio calculations of the H2-H2 potential reproted by Mulder, van der Avoird, and Wormer gives Δ/k = 0.0116 K which is close to the experimental value. This suggests that the static and dynamic effects of phonons on the splitting may have been overestimated in the past.

Infrared spectroscopic studies of photoinduced reactions of methyl radical in solid parahydrogen

Photolysis of methyl iodide in solid parahydrogen crystal at about 5 K by a low-pressure mercury lamp produces methyl radical, methane and ethane as evidenced by their infrared absorption spectra. It is found that even after the complete depletion of methyl iodide the intensity of the ν 3 and ν 4 bands of methane continues to increase with the concomitant decrease of the intensity of the ν 3 and ν 4 bands of methyl radica as logn as the mercury lamp illumination is continued. Possible mechanisms of the spectral changes are proposed.

Dephasing relaxation of J=2 rotons in parahydrogen crystals doped with hydrogen-deuterium impurities.

The dephasing relaxation times ${T}_{2}$ of the Raman-active J=2 rotons in para-${\mathrm{H}}_{2}$ crystals, intentionally doped with HD impurities, were measured by means of the time-resolved stimulated-Raman-gain (TRSRG) technique. The relaxation rates ${T}_{2}^{\mathrm{\ensuremath{-}}1}$ corresponding to the spectral linewidth \ensuremath{\delta}\ensuremath{\nu}=(\ensuremath{\pi}${T}_{2}$${)}^{\mathrm{\ensuremath{-}}1}$ are found to increase linearly with HD concentration with the slopes 0.148\ifmmode\pm\else\textpm\fi{}0.05 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, 0.126\ifmmode\pm\else\textpm\fi{}0.05 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, and 0.140\ifmmode\pm\else\textpm\fi{}0.05 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ per percent for the \ensuremath{\Vert}M\ensuremath{\Vert}=0, 1, and 2 rotons, respectively, which, within experimental error, are the same as in the case of ortho-${\mathrm{H}}_{2}$ impurities. This result demonstrates the equivalence of HD and ortho-${\mathrm{H}}_{2}$ for scattering of J=2 rotons in para-${\mathrm{H}}_{2}$. Furthermore the previously suggested possibility that the extrapolated values for the roton relaxation rate in pure para-${\mathrm{H}}_{2}$ would originate from HD impurities at the natural isotope concentration (${c}_{\mathrm{HD}\mathrm{\ensuremath{\approxeq}}0.03}$%) is eliminated.

Roton relaxation in parahydrogen crystals measured by time-resolved stimulated Raman gain.

Picosecond time-resolved dephasing measurements of the Raman-active J=2 rotons in para-${\mathrm{H}}_{2}$ single crystals have been performed with the stimulated-Raman-gain technique. The three distinct J=2 rotons (\ensuremath{\Vert}${M}_{J}$\ensuremath{\Vert}=0, 1, or 2) could be selected by a proper choice of the polarizations of the four laser beams with respect to the crystal axes. Excellent exponential fits were obtained for the decay curves, indicating little or no inhomogeneous broadening. The resulting ${T}_{2}$ times exhibit no temperature dependence between 5 and 10 K and are in good agreement with recent high-resolution spectral measurements. However, the present time-domain results are more reliable and accurate. In the explored range of ortho-${\mathrm{H}}_{2}$ impurity concentration of up to 5%, a linear increase is found for the relaxational linewidths (\ensuremath{\sim}1/${T}_{2}$) with nearly equal slopes for the different \ensuremath{\Vert}${M}_{J}$\ensuremath{\Vert} rotons. These results extrapolated to pure para-${\mathrm{H}}_{2}$ yield ${T}_{2}$ times in the neighborhood of 100 ps which agree quite well with those of a recent microscopic calculation. However, in the experiments unpredicted and significant differences (up to 30%) are evident between ${T}_{2}$ times for different \ensuremath{\Vert}${M}_{J}$\ensuremath{\Vert} values. The rotational relaxation was also measured in the liquid phase of para-${\mathrm{H}}_{2}$ and a value ${T}_{2}$=7.5 ps was obtained. It too showed a small decrease with increasing ortho-${\mathrm{H}}_{2}$ concentration.

Relaxation times of k=0 rotons in pure parahydrogen crystals and roton scattering by orthohydrogen impurities.

A high-resolution Raman investigation is presented of the linewidth of the $J=2$ rotons in solid $p\ensuremath{-}{\mathrm{H}}_{2}$ measured by means of a tandem interferometer-monochromator. The magnitude of the linewidths (\ensuremath{\cong} 0.08 ${\mathrm{cm}}^{\ensuremath{-}1}$) at extremely low $o\ensuremath{-}{\mathrm{H}}_{2}$ concentration is in good agreement with a recent microscopic calculation, but a variation of the experimental widths by a factor of 2 was observed for different $|{M}_{J}|$. The dependence of the roton linewidth on the concentration of $o\ensuremath{-}{\mathrm{H}}_{2}$ impurities is found to be linear, indicating that the $o\ensuremath{-}{\mathrm{H}}_{2}$ act as noninteracting random scattering centers.

Elastic constants of parahydrogen determined by Brillouin scattering

Five single crystals of parahydrogen were grown near the triple point, and their Brillouin spectra examined in known crystal directions. An analysis of the measured frequency shifts of longitudinal and transverse components led to the direct determination of the adiabatic elastic constants at 13.2 K: C11 = 3.34 ± 0.05, C13 = 0.56 ± 0.03, C33 = 4.08 ± 0.06, C44 = 1.04 ± 0.03 kbar. The fifth elastic constant C12 = 1.30 ± 0.05 kbar was evaluated from the relation C11 + C12 = C13 + C33. Values of the following constants were calculated: elastic anisotropy, A = 1.02 ± 0.06, adiabatic bulk modulus, Bs = 1.73 ± 0.04 kbar, and Debye temperature, θD = 111.2 K. All of these values were compared with earlier experimental measurements. The elastic constants were found to be in good agreement with a recent self-consistent phonon calculation by Goldman.

Hexadecapolar induced absorption in solid parahydrogen

We present a theoretical calculation of the intensities due to hexadecapolar induced absorption for the U1(0) and Q1(0) + U0(0) features and their associated phonon branches in a solid parahydrogen crystal. These results are compared with the recent experimental work of Prasad, Clouter, and Reddy and it is concluded that, within present uncertainties, the hexadecapolar mechanism accounts for the observed intensities.