Production of rotationally cold methyl radicals in pulsed supersonic beams

New Theoretical Infrared Line List for the Methyl Radical with Accurate Vibrational Band Origins from High-Level Ab Initio Calculations

In the present work, high-level ab initio calculations were carried out for the ground electronic state of the methyl radical (CH₃). Dunning's augmented correlation-consistent orbital basis sets were employed up to the quintuple-ζ valence quality with the core-valence electron correlation [aug-cc-pCV5Z] combined with the single- and double-excitation unrestricted coupled-cluster approach with a perturbative treatment of triple excitations [RHF-UCCSD(T)]. The explicitly correlated version of the coupled-cluster approach [RHF-UCCSD(T)-F12x{x = a, b}] was additionally applied with the core-valence cc-pCVQZ-F12 basis set in order to study convergence with respect to the basis set size. The contributions beyond the coupled-cluster level of the theory like Douglas-Kroll-Hess scalar relativistic Hamiltonian, diabatic Born-Oppenheimer corrections, and high-order electronic correlations have been included into the ab initio potential energy surfaces (PESs). It is shown that the theoretical band origins of CH₃ converge gradually to the experimental values when applying the ab initio PESs using the aug-cc-pCVXZ [X = T, Q, and 5] basis sets. For the first time, all available experimental band origins of the gaseous CH₃ are reproduced within an accuracy of 0.2 cm^-1 using a newly developed PES extrapolated to the complete basis set limit [CBS(TQ5Z)]. The reached accuracy is one order of magnitude better than that of the best available calculations. A new theoretical infrared line list was generated for astrophysical applications using an ab initio dipole moment surface computed at the RHF-UCCSD(T)/aug-cc-pCVQZ level of the theory. The manifestation of a large-amplitude motion in CH₃ is also discussed.

Achieving high molecular alignment and orientation for CH[Formula: see text]F through manipulation of rotational states with varying optical and THz laser pulse parameters

Increasing interest in the fields of high-harmonics generation, laser-induced chemical reactions, and molecular imaging of gaseous targets demands high molecular "alignment" and "orientation" (A&O). In this work, we examine the critical role of different pulse parameters on the field-free A&O dynamics of the CH[Formula: see text]F molecule, and identify experimentally feasible optical and THz range laser parameters that ensure maximal A&O for such molecules. Herein, apart from rotational temperature, we investigate effects of varying pulse parameters such as, pulse duration, intensity, frequency, and carrier envelop phase (CEP). By analyzing the interplay between laser pulse parameters and the resulting rotational population distribution, the origin of specific A&O dynamics was addressed. We could identify two qualitatively different A&O behaviors and revealed their connection with the pulse parameters and the population of excited rotational states. We report here the highest alignment of [Formula: see text] and orientation of [Formula: see text] for CH[Formula: see text]F molecule at 2 K using a single pulse. Our study should be useful to understand different aspects of laser-induced unidirectional rotation in heteronuclear molecules, and in understanding routes to tune/enhance A&O in laboratory conditions for advanced applications.

Identification of Isoprene Oxidation Reaction Products via Anion Photoelectron Spectroscopy

We present a study on the oxidation of isoprene under several different conditions that may model both atmospheric and combustion chemistry. Anions, formed by passing isoprene/oxidant gas mixtures through a pulsed discharge generating a range of species, are separated via mass spectrometry and characterized by anion photoelectron (PE) spectroscopy supported by computations. Specifically, a UV-irradiated isoprene/O₂ mixture, which additionally produces O₃, and an isoprene/O₂/H₂ mixture, which generates ^•OH when passed through the discharge, were sampled. The mass spectra of ions generated under both conditions show the production of intact molecular ions, ion-molecule complexes (e.g., O₂^-, O₄^-, and O₂^-·isoprene), and singly deprotonated species (e.g., deprotonated isoprene, C₅H₇^-). In addition, both smaller and oxidized fragments are observed using both gas mixtures, though relative abundances differ. From the UV-irradiated isoprene/O₂ gas mixture, additional intact molecular products of reactions initiated by ozonolysis of isoprene, methylglyoxal, and dimethylglyoxal were observed. Fragmentation and oxidation of isoprene observed in both gas mixtures included species with m/z 39, 53, 67, 69, and 83 that we attribute to a series of alkyl- and alkenoxide-based anions. The coexistence of intact molecules and complexes with fragments and reaction products demonstrates the versatility of this ion source as a simple and efficient anion formation method for studying species that may be relevant in atmospheric and combustion chemistry.

Development and characterization of high-repetition-rate sources for supersonic beams of fluorine radicals

We present and compare two high-pressure, high-repetition-rate electric-discharge sources for the generation of supersonic beams of fluorine radicals. The sources are based on dielectric-barrier-discharge (DBD) and plate-discharge units attached to a pulsed solenoid valve. The corrosion-resistant discharge sources were operated with fluorine gas seeded in helium up to backing pressures as high as 30 bars. We employed a (3 + 1) resonance-enhanced multiphoton ionization combined with velocity-map imaging for the optimization, characterization, and comparison of the fluorine beams. Additionally, universal femtosecond-laser-ionization detection was used for the characterization of the discharge sources at experimental repetition rates up to 200 Hz. Our results show that the plate discharge is more efficient in F₂ dissociation than the DBD by a factor between 8 and 9, whereas the DBD produces internally colder fluorine radicals.

Preparation of individual magnetic sub-levels of 4He(23S1) in a supersonic beam using laser optical pumping and magnetic hexapole focusing

We compare two different experimental techniques for the magnetic-sub-level preparation of metastable ⁴He in the 2³S₁ level in a supersonic beam, namely, magnetic hexapole focusing and optical pumping by laser radiation. At a beam velocity of v = 830 m/s, we deduce from a comparison with a particle trajectory simulation that up to 99% of the metastable atoms are in the M_J″ = +1 sub-level after magnetic hexapole focusing. Using laser optical pumping via the 2³P₂-2³S₁ transition, we achieve a maximum efficiency of 94% ± 3% for the population of the M_J″ = +1 sub-level. For the first time, we show that laser optical pumping via the 2³P₁-2³S₁ transition can be used to selectively populate each of the three M_J″ sub-levels (M_J″ = -1, 0, +1). We also find that laser optical pumping leads to higher absolute atom numbers in specific M_J″ sub-levels than magnetic hexapole focusing.