Hiromichi Hoshina

High Resolution Infrared Absorption Spectra of Methane Molecules Isolated in Solid Parahydrogen Matrices

We present high resolution (∼0.01 cm−1) infrared absorption spectra of the ν4 band of methane doped parahydrogen (CH4/pH2) solids produced by two different techniques: gas condensation in an enclosed cell at T≈8u200aK, and rapid vapor deposition onto a T≈2u200aK substrate in vacuum. The spectrum of the rapid vapor deposited solid contains a novel progression of single peaks with ≈5 cm−1 spacing, superimposed over the known spectrum of CH4 molecules trapped in sites of D3h symmetry in hexagonal close-packed (hcp) solid pH2. New theoretical calculations of the rovibrational transitions of a tetrahedral molecule in an external field of Oh symmetry permit the assignment of this new progression to CH4 molecules trapped in crystalline face centered cubic (fcc) regions of the pH2 solid. Annealing of the rapid vapor deposited samples to T≈5u200aK decreases the intensities of the CH4/pH2(fcc) absorptions, and results in intensity changes for parallel and perpendicularly polarized CH4/pH2(hcp) transitions. We discuss these phe...

Chemical reactions in quantum crystals

Solid parahydrogen, known as a quantum crystal, is a novel matrix for the study of physical and chemical processes of molecules at low temperatures by high-resolution infrared spectroscopy. Ro-vibrational motion of molecules embedded in solid parahydrogen is well quantized on account of the weak interaction in the crystal. In addition, molecules are almost free from the cage effect because of the softness of the quantum crystal, so that a variety of chemical reactions could be observed for molecules in solid parahydrogen at liquid He temperatures. In this article, we survey our recent studies on photodissociation of methyl radicals and subsequent reactions in solid parahydrogen, and discuss (1) the nuclear spin selection rule in chemical reactions and (2) pure tunnelling reactions obtained from the analysis of the present system. Contents PAGE 1.u2003 Introduction 534 2.u2003 Solid parahydrogen as a matrix 535 3.u2003 Experiments 536 4.u2003 Photochemistry of methyl iodide 537 5.u2003 Nuclear spin selection rule in chemical reactions 542 6.u2003 Tunnelling chemical reactions 546 7.u2003 Conclusion 550 Acknowledgement 550 References 550

Tunneling chemical reactions in solid parahydrogen: A case of CD3+H2→CD3H+H at 5 K

Ultraviolet photolysis of CD3I in solid parahydrogen at 5 K gives CD3 radical, which decreases in a single exponential manner with a rate constant of (4.7±0.5)×10−6u2009s−1. Concomitantly, CD3H is formed, which is accounted for by the quantum tunneling reaction CD3+H2→CD3H+H. Under the same conditions, CH3I yields CH3 radical, but the corresponding reaction between CH3 and H2, expected to give CH4+H, does not proceed measurably at 5 K. The difference between the two systems is attributed to the difference in the zero point energy change.

High-resolution laser spectroscopy of methane clusters trapped in solid parahydrogen

Clusters of methane are isolated in solid parahydrogen. The vibrational spectral region of the ν4 fundamental of methane molecule is surveyed with a Fourier transform infrared spectrometer and a high-resolution difference-frequency infrared laser system. More than 200 sharp absorption lines are discovered whose linewidth is as narrow as 0.007u2009cm−1 (200 MHz). The spectrum indicates that the rovibrational levels of the clusters of small sizes are well quantized in solid parahydrogen.

Rotation of methane and silane molecules in He droplets

This work studies the renormalization of the molecular moments of inertia I(G) in liquid helium. For this purpose we have measured the rotational-vibrational spectra of the nu(3) modes of a series of homologous light spherical top molecules such as CH(4), CD(4), SiH(4), and SiD(4) in He droplets. The spectra were fitted to an empirical gas phase Hamiltonian, yielding a set of spectroscopic constants. We found that the additional moment of inertia, DeltaI(He), scales approximately as square of I(G). This is in agreement with the theoretical model which assigns DeltaI(He) to coupling of molecular rotation with vibration of He in the molecular vicinity. Our results also indicate a large increase in the effective centrifugal distortion constants, which is another manifestation of the interaction of the molecular rotors with the He environment. Finally, the mechanism of the relaxation of rotational energy in liquid helium is discussed.

Internal Rotation of Methane Molecules in Large Clusters.

Methane is one of the very few substances that show rotation of individual molecules in the crystalline phase. Here we explore the evolution of the rotation spectrum of methane from single molecules to clusters containing up to about 4 × 10(3) molecules. The clusters were assembled in He droplets at T = 0.38 K and studied via infrared laser spectroscopy in the ν3 region of the methane molecules. Well-resolved rotational structure in the spectra was observed in clusters containing up to about 50 molecules. We have concluded that in distinction to the crystals molecular rotation in methane clusters is confined to the surface and is enabled by low coordination of the molecules. On the contrary the molecules in the clusters interior are in amorphous state wherein the rotation is quenched. These results demonstrate that even at very low temperature the surface of the methane clusters remains fluxional due to quantum effects.

Rotation of methane molecules in dimers and small clusters

This work reports on the study of the internal rotation of methane molecules in small clusters containing up to about five molecules. The clusters were assembled in helium droplets at T = 0.38 K by successive capture of single methane molecules and studied by infrared laser spectroscopy of the fundamental CH4u2009ν3 vibration around 3030 cm(-1). The spectra demonstrate well resolved structure due to internal rotation of the constituent molecules in the clusters. The most resolved spectrum for the dimers shows characteristic splitting of the lines due to anisotropic intermolecular interaction. The magnitude of the splitting is found to be in a good quantitative agreement with the recent theoretical anisotropic intermolecular potentials.

Ro-vibrational Spectra of (para-H2 )N -CH4 in He Droplets.

In this work, we report on the infrared spectroscopic study of clusters of CH4 molecules with up to N=80 para-hydrogen molecules assembled inside He droplets. Upon increase of the number of the added para-hydrogen molecules up to about N=12, both the rotational constant, B, and the origin frequency of the υ3 band of CH4 decrease gradually. In the range of 6 ≤N≤12, the spectra indicate some abrupt changes of B and υ3 with both values being approximately constant at N≥12. The origin of this effect is discussed. Comparison of the spectra of methane molecules in para-hydrogen clusters to that in solid para-hydrogen is also presented.