Direct measurement of fast ortho-para conversion of molecularly chemisorbed ${\mathrm{H}}_2$ on Pd(210)

Ortho-para conversion of molecularly chemisorbed ${\mathrm{H}}_2$ on a Pd(210) surface at a surface temperature of 50 K is reported. A combination of a pulsed molecular beam, photostimulated desorption, and resonance-enhanced multiphoton ionization techniques was used for probing the change in the rotational states of molecularly chemisorbed ${\mathrm{H}}_2$ on the surface. Our result shows that fast ortho-para conversion of chemisorbed ${\mathrm{H}}_2$ occurs. The conversion time was experimentally determined to be about 2 s, which is in good agreement with a previous theoretical calculation. This agreement supports that the ortho-para conversion mechanism of the molecularly chemisorbed ${\mathrm{H}}_2$ on Pd(210) is a two-step process based on the hyperfine-Coulomb excitation.

Figure 1

TOF spectra of photodesorbed ${\mathrm{H}}_2$ in $J=0–5$ from a Pd(210) surface at $T_{\mathrm{S}}=50$ K after 6 Langmuir of normal ${\mathrm{H}}_2$ exposure. The distance between the REMPI laser and the sample was 5 mm. The spectra were acquired by collecting the ${{\mathrm{H}}_2}^{+}$ signal while changing the delay time between the PSD and REMPI lasers.

Figure 2

Schematic diagram of the experimental setup and pulse sequence driving the molecular beam and two lasers for the $o-p$ conversion measurement. An ${\mathrm{H}}_2$ beam pulse is formed in a pulsed molecular beam expansion. After a skimmer ${\mathrm{H}}_2$ passes through a differential-pumping stage (not shown) and enters a UHV chamber. After a certain period of delay time (residence time) following the ${\mathrm{H}}_2$ deposition, the PSD laser at a repetition rate of 10 Hz photodesorbs ${\mathrm{H}}_2$ on the surface for 30 s (green line). The PSD laser irradiation onto the surface is controlled by mechanical shutter opening/closing. The photodesorbed ${\mathrm{H}}_2$ is state-selectively ionized by REMPI (light-purple line) and detected by an MCP. The delay time between the PSD and REMPI laser pulses corresponds to the TOF.

Figure 3

Intensities of photodesorbed ${\mathrm{H}}_2$ for $J=1$ detected by REMPI at different residence times: $t_{\mathrm{res}}=$ (a) 0.4 s, (b) 1 s, (c) 3 s, and (d) 20 s. These data points were obtained by averaging over the data from repeated measurements. The origin of the time axis corresponds to the opening of the pulsed valve of the molecular beam source. The representative of individual data of $t_{\mathrm{res}}=1$ s is shown in the inset.

Figure 4

Time evolution of the photodesorbed ${\mathrm{H}}_2$ intensities derived from the area intensities for the REMPI signals within 0.6 s after shutter opening, for $J=0$ (filled circle), 1 (open circle), 2 (filled square), 3 (open triangle), and 4 (filled triangle). The error bars represent the standard deviation of several measurements. For clarity, the data sets for the $J=2$, 3, and 4 are plotted separately from those of $J=0$ and $J=1$.

Figure 5

Time evolution of the photodesorbed ${\mathrm{H}}_2$ intensities for $o-{\mathrm{H}}_2$ (triangle), $p-{\mathrm{H}}_2$ (circle), and their sum (square). $o-{\mathrm{H}}_2$ and $p-{\mathrm{H}}_2$ intensities are derived from the summation of $J=1$ and $J=3$, and of $J=0, J=2$, and $J=4$ intensities shown in Fig. 4, respectively. The solid line for $o-{\mathrm{H}}_2$ is a fit using an exponential function, and that for $p-{\mathrm{H}}_2$ is based on the parameters obtained from the fitting of the $o-{\mathrm{H}}_2$ data.