Time-Resolved Study of Crystallization in Deeply Cooled Liquid Parahydrogen

We present real-time measurements of the crystallization process occurring in liquid para-hydrogen (para-${\mathrm{H}}_2$) quenched to $\approx 0.65T_m$ ($T_m=13.8\mathrm{K}$ is the melting point of bulk liquid para-${\mathrm{H}}_2$). The combination of high spatial resolution Raman spectroscopy and liquid microjet generation allows, in situ, capturing structural changes with $\sim {10}^{-8}\mathrm{s}$ time resolution. Our results provide a crystal growth rate that rules out a thermally activated freezing process and reveal that the quenched melt freezes into a metastable polymorph, which undergoes a structural transition. The achieved temporal control offers new opportunities for exploring the elementary processes of nonequilibrium phase transformations in supercooled liquids.

Figure 1

(a) Schematic view of the experimental setup (see the text). (b) Shadow image [24] of the continuous para-${\mathrm{H}}_2$ filament produced at a nominal source pressure of $\approx 14\mathrm{bar}$ and a source temperature of $\approx 15.8\mathrm{K}$. The white dashed line indicates the zero position of the glass capillary orifice, which is located about $30\mu \mathrm{m}$ to the right of the dark capillary edge visible in the image.

Figure 2

Selected Raman spectra of the rotational $S_0(0)$ (left) and vibrational $Q_1(0)$ (right) transitions recorded between $z\approx 0.03\mathrm{mm}$ and $z\approx 2.03\mathrm{mm}$. The uppermost rotational spectrum was measured further downstream from the orifice at $z\approx 2.53\mathrm{mm}$. The inset shows two examples of derivatives of vibrational spectra, whose outermost maxima and minima wave numbers define the temperatures of the coldest and hottest filament regions, respectively, by taking into account the spectral resolution. The top-axis vertical ticks in the left panel indicate the wave numbers of the experimental hcp [26] and theoretical fcc [14] rotational lines in bulk solid para-${\mathrm{H}}_2$.

Figure 3

Experimental liquid filament temperatures (circles, left $y$ axis) and solid fraction $f_S$ (squares, right $y$ axis) extracted from the vibrational spectra of Fig. 2. The purple circles represent the average temperature, whereas the red and blue symbols illustrate the highest and lowest temperatures, respectively. The solid lines are numerical simulation results of the cooling process (see the text). The dashed line marks the melting point of bulk liquid para-${\mathrm{H}}_2$. The time axis is defined as $t=z/v_{\mathrm{jet}}$, where $v_{\mathrm{jet}}$ is the jet velocity.

Figure 4

Relative peak area $f_{\mathrm{fcc}}$ (symbols) of the fcc rotational line at $\nu \approx 350{\mathrm{cm}}^{-1}$ with respect to the hcp line at $\nu \approx 351.2{\mathrm{cm}}^{-1}$ (Fig. 2). The areas are determined by fitting the experimental rotational lines to pseudo-Voigt functions. The dashed lines are guides to the eye emphasizing the slight change in the transformation rate at $z\approx 1.8\mathrm{mm}$, which roughly marks the end of freezing process (Fig. 3).