Microwave spectroscopy of deuterated molecular ions and structures of pyramidal XY3 molecules

荒木, 光典; アラキ, ミチノリ; ARAKI, Michinori

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Microwave spectroscopy of deuterated molecular ions and structures of pyramidal XY3 molecules

https://ir.soken.ac.jp/records/190

名前 / ファイル	ライセンス	アクション
要旨・審査要旨 / Abstract, Screening Result (574.9 kB)
本文 / Thesis (10.0 MB)

Item type

学位論文 / Thesis or Dissertation(1)

公開日

2010-02-22

タイトル

Microwave spectroscopy of deuterated molecular ions and structures of pyramidal XY3 molecules

タイトル

Microwave spectroscopy of deuterated molecular ions and structures of pyramidal XY3 molecules

言語

eng

資源タイプ

資源タイプ識別子

http://purl.org/coar/resource_type/c_46ec

資源タイプ

thesis

著者名

荒木, 光典

フリガナ

アラキ, ミチノリ

著者

ARAKI, Michinori

学位授与機関

学位授与機関名

総合研究大学院大学

学位名

博士（理学）

学位記番号

内容記述タイプ

Other

内容記述

総研大甲第374号

研究科

値

数物科学研究科

専攻

値

07 構造分子科学専攻

学位授与年月日

1999-03-24

学位授与年度

値

1998

要旨

内容記述タイプ

Other

内容記述

Over one hundred molecular species have been detected in interstellar space by radio astronomical observations. It is generally accepted that the most abundant molecules in dark clouds have been already detected. These interstellar molecules are mainly produced by ion-molecule reactions, which are uniquely efficient in the interstellar physical conditions of low temperature and low density. This makes molecular ions extremely important intermediates in the interstellar chemistry even at very small abundances. Since detection of ions in interstellar space has been extremely limited, it is thought that they have not accumulated sufficient observational information to explain the reaction scheme. Physical and chemical processes in the interstellar space can be studied by using spectral lines of the molecular ions, and laboratory microwave spectroscopy is a powerful tool for supplying precise transition frequencies of the ions to radio astronomy. However, laboratory measurements are not easy because of the difficulty in producing sufficient concentrations of ions. Development of efficient production methods for molecular ions will make it possible to observe many more of them by microwave spectroscopy.
Several interstellar molecules have been found to have large D/H ratios enhanced by factors of 103- 104 over the primordial cosmic D/H ratio of ? 1.5 X 10-5. For example, the abundance ratio of the deuterated species to HCN, [DCN]/ [HCN], is found to be 0.023 in the dark cloud, Taurus Molecular Cloud (TMC-1). This enhancement is called "deuterium enrichment." The degree of the deuterium enrichment is determined by physical conditions and related reactions in interstellar clouds. In other words the deuterium enrichment is generally a good probe to study the reaction scheme in space. As a result laboratory microwave spectroscopic studies of deuterated species are important in astronomy as well as in molecular spectroscopy. He has studied several deuterated molecular ions related to interstellar phenomena.

Microwave spectra of D3O+ and D3S+
The hydronium ion H3O+ is a precursor to fundamental hydroxy interstellar molecules. For example, OH and H2O are generated by an exothermic recombination of electrons with H3O+. Similarly H2DO+ is a precursor to OD and HDO and is generated by a series of ion-molecule reactions starting from a reaction of O with H2D+. H2D+ is a key molecule in the interstellar deuterium fractionation processes. Therefore, the interstellar [H2DO+]/[H3O+] ratios are very important information for an understanding of the fractional reaction scheme in molecular clouds. Although the inversion-rotation spectrum of H3O+ is well known, prediction of H2DO+ is not easy because it requires a precise molecular structure of H3O+ and a precise potential for inversion motion of H3O+. So, to obtain an improved the molecular structure and potential of H3O+, he studied the inversion-rotation spectrum of its fully deuterated species D3O+.
Microwave spectra of the ions were observed using a microwave spectrometer at the Institute for Molecular Science. The spectrometer was a 100 kHz source- modulated system. Microwave radiation was generated with a combination of frequency multipliers and klystrons. An InSb photoconductive detector cooled by liquid helium was used to measure the power of microwave radiation. A free space discharge cell was cooled by circulating liquid nitrogen through a copper tube soldered on a copper sheet covering the glass cell.
After several trials he found that deuterated species of H3O+ were efficiently produced by a hollow-cathode dc-glow discharge in a mixture of D2 and D2O in a 2 m length and 10 cm diameter free-space absorption cell. The length of the hollow cathode was 1.3 m. The production of the ions in the cell is increased by the hollow cathode effect.
Fifty three P- and Q-branch transitions between the lowest pair of levels of the inversion motion were precisely measured in the frequency region of 220 to 565 GHz. An analysis of the observed spectral lines yielded molecular constants for the upper (0-) and lower (0+) levels in the inversion motion. The inversion splitting was accurately determined to be 15.35550086(147) cm-1, where the number in parentheses denotes one standard deviation of the fit. Combined with IR data, the inversion splitting of D3O+ for the v2 inversion state was determined to be 191.38874(98) cm-1. As a total, two inversion splittings were precisely determined by microwave spectroscopy and are important information for a proper understanding of the inversion potential of H3O+. Potential function parameters can be revised using the two inversion splittings. Furthermore, a reliable inversion splitting of H2DO+ can now be estimated from the new potential function parameters and used to predict transition frequencies of H2DO+.
The average structures (rz structure) of H3O+ and D3O+, that is, averaged for zero point motion in the ground vibrational state, were derived from their averaged rotational constants. The results are as follows: for D3O＋ (0+) ; rz = 0.98392(152) Å, θz = 113.62(65)°, and for D3O+ (0-) ; rz = 0.98510(152)Å,θz = 112.94(65)° where r is O-D bond length and θ D-O-D bond angle. The equilibrium structure (re structure) of H3O+, a hypothetical structure of the potential, was determined from the rz structures of H3O+ and D3O+ by extrapolations. The re structure was obtained to be re = 0.9780(59) Å,θe. = 112.8(25)°. As a result, he was able to estimate the rotational constants of H2DO+ from the structures of H3O+ and D3O+, which were used to predict a spectral pattern of H2DO+.
In addition, he succeeded in observing the D3S+ ion generated by the hollow- cathode discharge. The J= 1-0 to 4-3 spectral lines of D3S+ were measured in the 152 - 610 GHz region. The molecular constants were determined from the measured frequencies. The precise molecular structure of H3S+ and D3S+ was derived from their rotational constants.

Microwave spectra of HCNH+ and its deuterated species
The method of a magnetically confined dc-glow discharge was employed to efficiently produce a protonated ion HCNH+, whose concentration was enhanced due to a lengthening of ion-rich negative glow and an increase of ionizing electron density. Around the absorption cell is wound a solenoid coil of enameled wire. The magnetic field was generated to confine the discharge plasma. The HCNH+ and its isotopic species were produced by the discharge in HCN and/or DCN at around -120℃. The optimum magnetic field was about one hundred Gauss.
The interstellar protonated hydrogen cyanide ion, HCNH+, is mainly produced by reactions of HNC and HCN with H3+ HCN+ or H3O+, and is also considered to be a precursor of HNC, HCN and CN in dark molecular clouds. Detection of HCND+ may give information about the deuterium fractionation in molecular clouds. The pure rotational transitions of HCNH+, and its isotopic species, HCND+ and DCND+, were measured in the 107 - 482 GHz region. The rotational constant B0 and the centrifugal distortion constant D0 for each ion were precisely determined by a least- squares fitting to the observed spectral lines. The observed rotational transition frequencies and the predicted ones are accurate within about 30 to 40 kHz and are useful for astronomical searches of HCNH+ and HCND+.

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Microwave spectroscopy of deuterated molecular ions and structures of pyramidal XY3 molecules

× 荒木, 光典

× アラキ, ミチノリ

× ARAKI, Michinori

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