Experimental and theoretical studies on vacuum ultraviolet absorption cross sections and photodissociation of CH3OH, CH3OD, CD3OH, and CD3OD

Bing Ming Cheng*, Mohammed Bahou, Wei Cheng Chen, Chin Hui Yui, Yuan-Pern Lee, L. C. Lee

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

60 Scopus citations

Abstract

Absorption cross sections of CH3OH,CH3OH, CH3OD,CH3OD, CD3OH,CD3OH, and CD3ODCD3OD are measured in a 107–220 nm <named-content content-type="sem:AIPTh1.2" rid="kwd1.4 kwd1.7" style="font-family: Lora, serif; font-size: 20px;">spectral</named-content> region using <named-content content-type="sem:AIPTh1.2" rid="kwd1.5" style="font-family: Lora, serif; font-size: 20px;">synchrotron radiation.</named-content> <named-content content-type="sem:AIPTh1.2" rid="kwd1.4" style="font-family: Lora, serif; font-size: 20px;">Spectra</named-content> of improved quality for four deuterated isotopomers, coupled with extensive calculations on low-lying <named-content content-type="sem:AIPTh1.2" rid="kwd1.2" style="font-family: Lora, serif; font-size: 20px;">excited states</named-content> of methanol using <named-content content-type="sem:AIPTh1.2" rid="kwd1.3" style="font-family: Lora, serif; font-size: 20px;">time-dependent density functional theory</named-content> with a large cc-pV5Z basis set, enable us to improve assignments of observed <named-content content-type="sem:AIPTh1.2" rid="kwd1.4 kwd1.7" style="font-family: Lora, serif; font-size: 20px;">spectral</named-content> features and to better understand the nature of these electronic transitions. Energies and <named-content content-type="sem:AIPTh1.2" rid="kwd1.6" style="font-family: Lora, serif; font-size: 20px;">oscillator</named-content> strengths of all transitions predicted with calculations are consistent with experimental results. Observed <named-content content-type="sem:AIPTh1.2" rid="kwd1.8" style="font-family: Lora, serif; font-size: 20px;">isotopic shifts</named-content> clearly indicate that absorption features in the 163–220 nm region (transition 1 1A″–X 1A′)1 1A″–X 1A′) are associated mainly with breaking of the O–H bond, consistent with <named-content content-type="sem:AIPTh1.2" rid="kwd1.3" style="font-family: Lora, serif; font-size: 20px;">theoretical</named-content> predictions. In the 151–163 nm region (transition 2 1A″–X 1A′),2 1A″–X 1A′), observed small vibrational spacings (806 cm−1 for CH3OH)CH3OH) associated with the C–O stretching mode can be rationalized with a broad double-well-like potential-energy curve resulting from avoided crossing of <named-content content-type="sem:AIPTh1.2" rid="kwd1.1" style="font-family: Lora, serif; font-size: 20px;">Rydberg states</named-content> 11A″(2a″→3s)11A″(2a″→3s) and 2 1A″(2a″→3p);2 1A″(2a″→3p); with isotopic data, further vibrational progressions are identified. Absorption lines in the 140–151 nm region with regular vibrational spacing (∼1046 cm−1 for CH3OH),CH3OH), likely associated with the CH2CH2 twisting mode, are assigned to nearly degenerate transitions 3 1A″–X 1A′3 1A″–X 1A′ and 3 1A′–X 1A′;3 1A′–X 1A′; the 3 1A″3 1A″ and 3 1A′3 1A′ states are associated with <named-content content-type="sem:AIPTh1.2" rid="kwd1.2" style="font-family: Lora, serif; font-size: 20px;">excitations</named-content> 2a″→3p′2a″→3p′ and 2a″→3p″,2a″→3p″, respectively. Progressions associated with the torsional mode of the <named-content content-type="sem:AIPTh1.2" rid="kwd1.2" style="font-family: Lora, serif; font-size: 20px;">excited state</named-content> are observed for the first time. For wavelengths smaller than 140 nm, <named-content content-type="sem:AIPTh1.2" rid="kwd1.1" style="font-family: Lora, serif; font-size: 20px;">Rydberg</named-content> transitions with n⩾3n⩾3 are tentatively assigned in accord with their quantum defects that are identical for all isotopomers in each <named-content content-type="sem:AIPTh1.2" rid="kwd1.1" style="font-family: Lora, serif; font-size: 20px;">Rydberg</named-content> series.
Original languageEnglish
Pages (from-to)1633-1640
Number of pages8
JournalJournal of Chemical Physics
Volume117
Issue number4
DOIs
StatePublished - 22 Jul 2002

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