Bor-Chen Chang

New electronic spectra of the HCCl and DCCl Ã-X̃ vibronic bands

The dispersed fluorescence spectra following the excitation of several A<--X vibronic bands of HCCl and DCCl at visible wavelengths were successfully acquired in a discharge supersonic free jet expansion using an intensified charge-coupled device detector. The dispersed fluorescence spectra reveal more details of the X(1) A() state vibrational structure in these molecules than previous reports. Dispersed fluorescence spectra of all four isotopomers (HC(35)Cl, HC(37)Cl, DC(35)Cl, and DC(37)Cl) were obtained. These dispersed fluorescence spectra exhibit the vibrational structures up to approximately 6000 cm(-1) above the zero-point level and determine the vibrational structures of HC(37)Cl and DC(37)Cl. Complete vibrational parameters including fundamental frequencies, anharmonicities, and coupling constants were determined for the HCCl/DCCl X(1) A() state. Furthermore, perturbations from the background triplet state a(3) A() and emission to triplet state levels were observed in the spectra. The singlet-triplet energy gap from the zero-point level could be determined to be 2167 cm(-1) (6.20+/-0.05 kcal/mol) in HCCl and to be 2187 cm(-1) (6.25+/-0.05 kcal/mol) in DCCl. Additionally, some of the A<--X excitation spectrum are reported for HCCl and DCCl.

Laser excitation and dispersed fluorescence spectra of the HCBr ÖX̃ vibronic transition

We recorded the laser-induced fluorescence (LIF) excitation spectra of several HCBr A–X vibronic transitions between 580 nm and 630 nm in a direct current (DC) discharge supersonic free jet expansion. Dispersed fluorescence spectra, following excitation of these HCBr A–X vibronic transitions were successfully obtained for the first time. The analysis of the dispersed fluorescence spectra reveals the details of the vibrational structure of the Xu200a1A′ state as well as that of the au200a3A″ state. The vibrational frequencies were determined as 1119 cm−1 (bend), 676 cm−1 (C–Br stretch) for the Xu200a1A′ state and 994 cm−1 (bend), 733 cm−1 (C–Br stretch) for the au200a3A″ state. In addition, based upon the observation of the triplet state levels, the value of the triplet–singlet energy gap (the zero-point level energy gap) could be extracted from the dispersed fluorescence spectra and is to be approximately 2006±8u200acm−1u200a(5.73±0.02u200akcal/mol).

Dispersed fluorescence spectrum of the vibronic transition

Abstract The dispersed fluorescence spectrum following the excitation of HC 35 Cl A – X vibronic transition between 570 and 610 nm was successfully obtained in a direct current (DC) discharge supersonic free jet expansion. The analysis of the dispersed fluorescence spectrum results in detailed information of the HC 35 Cl X 1 A ′ state vibrational structure. The bending and C–Cl stretching frequencies as well as anharmonicity constants were determined for the first time. In addition, since no perturbation was observed in the vibrational structure of the X 1 A ′ state up to 2788 cm −1 , a lower limit of triplet–singlet energy gap was established to be roughly 8.0 kcal/mol.

Experimental study of the DCCl X̃1A′ state vibrational structure by dispersed fluorescence spectroscopy

Dispersed fluorescence spectra following excitation of the DCCl A-X vibronic bands between 570 and 620 nm were successfully recorded for the first time using a combination of direct current discharge and supersonic free jet expansion techniques to produce the radical. This is the first experimental study on the DCCl X1A′ state vibrational structure. Analysis of the dispersed fluorescence spectra reveals details of the vibrational structure of the DCCl X1A′ state. Emission bands to all three vibrational modes were observed and the fundamental vibrational frequencies were determined: 2078 cm−1 (C–D stretch), 893 cm−1 (bend), and 801 cm−1 (C–Cl stretch). Vibrational parameters including anharmonicities and the bend-stretch coupling constant for the DCCl X1A′ state were also determined using a least-square fit. In addition, based on our results, the triplet-singlet energy gap (the zero-point level energy gap) could be estimated to be approximately 11±2 kcal/mol.

Hot bands in the spectrum of HCBr

Abstract Spectra of both the A(0,xa00,xa00)–X(0,xa01,xa00) and A(0,xa00,xa00)–X(0,xa00,xa01) bands in the A–X band system of HCBr have been recorded and rotationally assigned. The HCBr radical was formed by 193xa0nm excimer laser photolysis of bromoform. We found no signals from HCBr when the photolysis laser wavelength was changed to 248xa0nm, although strong CH product emission was observed by eye at both wavelengths. Most of the spectra were recorded in absorption at ambient temperature in a long-path absorption cell using a single frequency Ti:sapphire laser as the light source, but some congested sections were recorded under jet-cooled conditions. Analysis yielded accurate values for the vibrational fundamentals and rotational constants for the vibrationally excited ground state levels determined. A combination of experimental measurements and ab initio vibration–rotation constants was used to estimate equilibrium parameters and a ground state structure for HCBr.

Dispersed fluorescence spectra of the CCl2Ö vibronic bands

Laser excited, dispersed fluorescence spectra have been recorded for a number of bands of the A– electronic transition of CCl2. Analysis of the data yields well defined values for the harmonic and anharmonic constants for the symmetric ground state vibrations. Additional lines are observed at red shifts ≥5000 cm−1 from the origin, which are not assignable in terms of the known and A state structure. A possible explanation for these transitions would be perturbations of the singlet structure by the low-lying triplet a state.

Electronic spectroscopy of bromomethylenes in a supersonic free jet expansion

Laser excitation and dispersed fluorescence spectra of bromomethylenes (CBr2 and HCBr) between 546 and 576 nm have been recorded following their production in a plasma discharge in a supersonic free jet expansion. The analyses of the excitation and dispersed fluorescence spectra reveal several new vibrational levels of the CBr2 nA state as well as the detailed vibrational structure of the state. In addition, a new A− band of HCBr was found in the excitation spectrum and the dispersed fluorescence spectrum following the excitation of this new HCBr band has also been recorded. The complete vibrational structure of the HCBr u20061A′ state including the previously unknown C–H stretch fundamental frequency was determined for the first time and the rotational constants for three HCBr bands at visible wavelengths were also determined for the first time.

CH2 b̃1B1−ã1A1 Band Origin at 1.20 μm

The origin band in the b̃(1)B(1)-ã(1)A(1) transition of CH(2) near 1.2 μm has been recorded at Doppler-limited resolution using diode laser transient absorption spectroscopy. The assignments of rotational transitions terminating in upper state levels with K(a) = 0 and 1, were confirmed by ground state combination differences and extensive optical-optical double resonance experiments. The assigned lines are embedded in a surprisingly dense spectral region, which includes a strong hot band, b̃(0,1,0) K(a) = 0 - ã(0,1,0) K(a) = 1 sub-band lines, with combination or overtone transitions in the ã(1)A(1) state likely responsible for the majority of unassigned transitions in this region. From measured line intensities and an estimate of the concentration of CH(2) in the sample, we find the transition moment square for the 0(00) ← 1(10) transition in the b̃(1)B(1)(0,0,0)(0)-ã(1)A(1)(0,0,0)(1) sub-band is 0.005(1) D(2). Prominent b̃(1)B(1)(0,1,0)(0)-ã(1)A(1)(0,1,0)(1) hot band lines were observed in the same spectral region. Comparison of the intensities of corresponding rotational transitions in the two bands suggests the hot band has an intrinsic strength approximately 28 times larger than the origin band. Perturbations of the excited state K(a) = 0 and 1 levels are observed and discussed. The new measurements will lead to improved future theoretical modeling and calculations of the Renner-Teller effect between the ã and b̃ states in CH(2).

Dispersed fluorescence spectroscopy of the GeCl2 Ã-X transition

The laser-induced fluorescence excitation spectrum of the GeCl(2) A-X transition at ultraviolet wavelengths (300-320 nm) was recorded in a direct current discharge supersonic free jet expansion. The excitation spectrum contains several sharp peaks and a congested diffuse structure. Dispersed fluorescence spectra following the excitation of these GeCl(2) ultraviolet bands were successfully acquired for the first time. The analysis of the dispersed fluorescence spectra reveals the detailed vibrational structure of the X (1)A(1) state. We have assigned the vibrational structures corresponding to different isotopomers (Ge(35)Cl(2), Ge(35)Cl(37)Cl, and Ge(37)Cl(2)). The vibrational fundamental frequencies were determined: 409 cm(-1) (symmetric stretch), 159 cm(-1) (bend), and 352 cm(-1) (antisymmetric stretch) for the X (1)A(1) state of GeCl(2). Vibrational parameters of the ground electronic state including vibrational frequencies, anharmonicity, and bend-stretch coupling constant were determined. Our dispersed fluorescence spectra also clarify the vibrational assignments of the hot bands and provide more experimental data for unraveling the nature of the congested diffuse structure at shorter wavelengths in the excitation spectrum.