以霍式轉換紅外光譜儀及可調頻之紅外雷射研究para-H2間質中Cl + H2 → HCl + H反應

Abstract

本實驗利用霍式轉換紅外光譜儀(FTIR)以及可調頻之紅外雷射觀測Cl原子在para-H2(p-H2)間質中的紅外吸收光譜以及HCl作用光譜，研究Cl/p-H2間質光區4000—4750 cm-1中各個吸收峰對於反應Cl(2P3/2) + H2(v, J) → HCl + H的相對效率。觀測Cl/p-H2間質及其隨光譜儀內建光源照射延遲時間消長的吸收光譜指派4148.6及4151.0 cm-1為Cl微擾所引致的Q1(0)吸收峰，而4476.5及4479.2 cm-1為Cl微擾所引致的S1(0)吸收峰。由於在Q1(0)及S1(0)光區中皆僅觀測到兩個吸收峰，吾人推測其源於共線及T形Cl—H2雙體結構。且Cl與H2的反應過程經過一線性的過渡態[Cl-H-H]┿，依照可調頻雷射激發此四個吸收峰所得之HCl相對效率大小，吾人認為4148.6及4151.0 cm-1分別由共線及T形之Cl—H2雙體結構的Q1(0)躍遷，且4476.5及4479.2 cm-1為共線及T形之Cl—H2雙體結構的S1(0)躍遷。藉由可調頻雷射激發並比較4000—4750 cm-1中各個吸收峰所得的HCl相對效率發現H2的轉動運動會干擾其伸張振動與Cl進行反應，且由Cl活化的H2吸收峰比p-H2固體吸收峰容易產生HCl。當365 nm光源光解局部Cl2/p-H2間質後，在遠離光解區域之雷射激發實驗證實了H2的QR(0)吸收峰為非定域(delocalized)躍遷，而H2的Q1(0)–Cl吸收峰則為定域(localized)躍遷。

Excitation of the solid p-H2 with IR light in the range 4000–4750 cm-1 from a globar source induces reactions of Cl atoms with p-H2 to form HCl. We investigated the reaction of Cl with solid p-H2 with a tunable IR laser and a FTIR spectrometer. The Cl atoms were produced on irradiation of a Cl2/p-H2(1/1600) matrix with light at 365 nm from a light-emitting diode. By monitoring continuously the formation of HCl while tuning the wavelength of the IR laser, we obtained the relative yield of HCl as a function of excitation wavelength. In the HCl action spectra, 4148.6 and 4151.0 cm-1 were observed for Cl perturbed Q1(0) bands, meanwhile, 4476.5 and 4479.2 cm-1 were observed for Cl perturbed S1(0) bands. Comparison of the relative efficiency among different absorption peaks in the range of 4000–4750 cm-1 gave that the rotational motion of H2 molecules would hinder the reaction of Cl + H2 → HCl + H, which went through a linear transition state. In separate experiments we irradiated a selective area of the matrix with UV light and compared the yields of HCl in this area when we irradiated IR light at various locations. When the IR laser light irradiated outside the UV-irradiated area, excitation of p-H2 near 4229 cm-1, but not the Cl-H2 complex at 4148.5 cm-1, still produced HCl. In contrast, when the IR light irradiated inside the UV-irradiated region, more HCl was produced at 4148.5 cm-1. This indicates that the excited H2 can move around and react with Cl located far away from the originally excited area, whereas the IR excited Cl-H2 complex reacts directly to form HCl. We also found that irradiation of H2 produced some dimers of HCl, whereas irradiation at the Cl-H2 complex produced only HCl monomer.