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High-resolution infrared spectroscopy of jet cooled CH <sub/>2</sub> Br radicals: The symmetric CH stretch manifold and absence of nuclear spin cooling
摘要： Direct laser absorption of a slit supersonic discharge expansion provides the first high-resolution spectroscopic results on the symmetric CH stretch excitation (ν1) of the bromomethyl (CH2Br) radical in the ground electronic state. Narrowband (<1 MHz) mid-infrared radiation is produced by difference-frequency generation of two visible laser beams, with the open shell halohydrocarbon radical generated by electron dissociative attachment of CH2Br2 in a discharge and rapidly cooled to Trot = 18 ± 1 K in the subsequent slit-jet supersonic expansion. A rovibrational structure in the radical spectrum is fully resolved, as well as additional splittings due to spin-rotation effects and 79Br/81Br isotopologues in natural abundance. Spectroscopic constants and band origins are determined by fitting the transition frequencies to a non-rigid Watson Hamiltonian, yielding results consistent with a vibrationally averaged planar radical and an unpaired electron in the out-of-plane pπ orbital. Additionally, extensive satellite band structure from a vibrational hot band is observed and analyzed. The hot band data is compared to CFOUR/VPT2 (CCSD(T)cc-pVQZ) ab initio anharmonic predictions of the vibration rotation alpha matrix, which permits unambiguous assignment to CH2 symmetric-stretch excitation built on the singly excited CH2 out-of-plane bending mode (ν1 + ν4 ← ν4). Longitudinal cooling of the Doppler width in the slit-jet expansion geometry also reveals partially resolved hyperfine structure on transitions out of the lowest angular momentum states in excellent agreement with predictions based on microwave studies. High level ab initio MOLPRO calculations [CCSD(T)-f12b/VnZ-f12 (n = 3, 4, CBS)] are also performed with explicitly correlated f12 electron methods for the out-of-plane CH2 bending mode over the halogen series CH2X (X = F, Cl, Br, I), which clearly reveals a non-planar geometry for X = F (with a ΔE ≈ 0.3 kcal/mol barrier) and yet planar equilibrium geometries for X = Cl, Br, and I. Finally, a detailed Boltzmann analysis of the transition intensities provides support for negligible collisional equilibration of the entangled H atom nuclear spin states on the few hundred microsecond time scale and high collision densities of a slit supersonic expansion.
关键词： infrared spectroscopy,jet cooled CH2Br radicals,symmetric CH stretch manifold,nuclear spin cooling
更新于2025-09-23 15:21:01
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Locating Cytosine Conical Intersections by Laser Experiments and <i>Ab Initio</i> Calculations
摘要： The decay mechanism of S1 excited cytosine (Cyt) and the effect of substitution are studied combining jet-cooled spectroscopy (nanosecond resonant two-photon ionization (R2PI) and picosecond lifetime measurements) with CASPT2//CASSCF computations for eight derivatives. For Cyt and five derivatives substituted at N1, C5, and C6, rapid internal conversion sets in at 250?1200 cm?1 above the 000 bands. The break-off in the spectra correlates with the calculated barriers toward the "C5?C6 twist" conical intersection, which unambiguously establishes the decay mechanism at low S1 state vibrational energies. The barriers increase with substituents that stabilize the charge shifts at C4, C5, and C6 following (1ππ*) excitation. The R2PI spectra of the clamped derivatives 5,6-trimethyleneCyt (TMCyt) and 1-methyl-TMCyt (1M-TMCyt), which decay along an N3 out-of-plane coordinate, extend up to +3500 and +4500 cm?1.
关键词： CASPT2//CASSCF computations,jet-cooled spectroscopy,internal conversion,substitution effect,R2PI spectra,decay mechanism,cytosine,conical intersection
更新于2025-09-23 15:19:57
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Electronic State and Photophysics of 2-Ethylhexyl-4-methoxycinnamate as UV-B Sunscreen under Jet-Cooled Condition
摘要： The title compound, 2-ethylhexyl-4-methoxycinnamate (2EH4MC), is known as a typical ingredient of sunscreen cosmetics that effectively converts the absorbed UV-B light to thermal energy. This energy conversion process includes the nonradiative decay (NRD): trans?cis isomerization and finally going back to the original structure with a release of thermal energy. In this study, we performed UV spectroscopy for jet-cooled 2EH4MC to investigate the electronic/geometrical structures as well as the NRD mechanism. Laser-induced-fluorescence (LIF) spectroscopy gave the well-resolved vibronic structure of the S1?S0 transition; UV?UV hole-burning (HB) spectroscopy and density functional theory (DFT) calculations revealed the presence of syn and anti isomers, where the methoxy (?OCH3) groups orient in opposite directions to each other. Picosecond UV?UV pump?probe spectroscopy revealed the NRD process from the excited singlet (S1 (1ππ*)) state occurs at a rate constant of ～1010?1011 s?1, attributed to internal conversion (IC) to the 1nπ* state. Nanosecond UV?deep UV (DUV) pump?probe spectroscopy identified a transient triplet (T1 (3ππ*)) state, whose energy (from S0) and lifetime are 18 400 cm?1 and 20 ns, respectively. These results demonstrate that the photoisomerization of 2EH4MC includes multistep internal conversions and intersystem crossings, described as "S1 (trans, 1ππ*) → 1nπ* → T1 (3ππ*) → S0 (cis)".
关键词： nonradiative decay,photophysics,DFT calculations,trans?cis isomerization,jet-cooled spectroscopy,2-ethylhexyl-4-methoxycinnamate,UV-B sunscreen
更新于2025-09-19 17:13:59
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Revealing Long-Range Substituent Effects in the Laser-Induced Fluorescence and Dispersed Fluorescence Spectra of Jet-Cooled CH <sub/><i>x</i> </sub> F <sub/> 3– <i>x</i> </sub> CH <sub/>2</sub> O ( <i>x</i> = 1, 2, 3) Radicals
摘要： The B?←X? laser-induced fluorescence (LIF) and dispersed fluorescence (DF) spectra of the atmospherically important β-monofluoro ethoxy (MFEO), β,β-difluoro ethoxy (DFEO), and β,β,β-trifluoro ethoxy (TFEO) radicals were recorded with vibronic resolution under jet-cooled conditions. To simulate the spectra, Franck-Condon factors were obtained from quantum chemical computations carried out at the CAM-B3LYP/6-311++G(d,p) level of theory. The simulations reproduce well both the LIF and DF spectra. Both conformers (G and T) of MFEO and one (G) of the two conformers of DFEO contribute to the LIF spectrum. A comparison between the experimental and calculated spectra confirms the expected long-range field effects of the CHxF3?x group on electronic transition energies and bond strengths, especially in the excited electronic (B?) state. Although TFEO has only one conformer, its LIF spectrum is highly congested, which is attributed to the interaction between CO stretch and the ?CF3 internal rotation.
关键词： Laser-Induced Fluorescence,Molecular Structure,Spectroscopy,Quantum Chemistry,Fluoroethoxy Radicals,Jet-cooled,Dispersed Fluorescence
更新于2025-09-12 10:27:22
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Laser-Induced Fluorescence and Dispersed Fluorescence Spectroscopy of Jet-Cooled Isopentoxy Radicals
摘要： The B?-X? laser-induced fluorescence (LIF) and dispersed fluorescence (DF) spectra of jet-cooled isopentoxy radicals have been obtained. The LIF spectrum of isopentoxy lacks strong transitions to the CO-stretch levels that are typical for alkoxy radicals. Instead, it contains two low-frequency vibrational progressions due to large-amplitude motions of the GG’ and GG conformers involving torsion of the C1C2C3H dihedral angle. Other vibronic bands observed in the LIF spectrum are attributed to the TG conformer. Molecular carriers of the vibronic transitions in the LIF spectrum are identified by comparing the experimentally obtained spectrum and the simulated one. DF spectra of the GG and TG conformers are dominated by strong vibrational progressions of the CO-stretch mode when the origin or the CO-stretch band is pumped. When non-CO-stretch bands are pumped, the DF spectra are dominated by combination bands of the CO-stretch and the pumped mode. ?-X? separations of the GG, and TG conformers were also determined from the DF spectra.
关键词： Isopentoxy Radicals,Conformers,Laser-Induced Fluorescence,Vibrational Progressions,Jet-Cooled,Dispersed Fluorescence Spectroscopy
更新于2025-09-11 14:15:04