The jet-cooled stibino (SbH₂) free radical has been detected for the first time. This highly reactive species was produced in an electric discharge through a precursor mixture of stibine (SbH₃) diluted in high pressure argon. Stibine was synthesized by the low-temperature reduction of SbCl₃ with LiAlH₄ and stored and handled at -85 ^oC to avoid decomposition. Low-resolution LIF scans revealed a single band of SbH₂ with complex rotational structure in the 514.9 - 511.0 nm region. We find that the fluorescence lifetimes of the rotational transitions in this band are very short, of the order of 50-75 ns, suggesting an upper state dissociative process. The spectrum is assigned to the òA₁ - ~X²B₁ electronic transition by analogy with the known spectra of NH₂, PH₂ and AsH₂ and in accord with a recent high level ab initio study. Emission spectra obtained after laser excitation of single rotational lines in the 0-0 band show a ground state bending frequency of approx. 820 cm⁻¹, consistent with theoretical predictions. The rotationally resolved spectrum of the 0-0 band, which spans some 150 cm⁻¹, was recorded at a resolution of 0.08 cm⁻¹and analyzed in detail. The spectrum is complicated by large spin splittings and Sb hyperfine effects. The molecular constants were used to determine the geometry of SbH₂ in both states.

Isoprene is the most abundant volatile organic compound (VOC) in the Earth’s atmosphere after methane. Ozonolysis of isoprene, with the production of the Criegee intermediate methyl vinyl ketone oxide (MVKO), plays an important role in atmospheric chemistry. Recently, Barber et al.V. P. Barber et al. J. Am. Chem. Soc., 140, 10866-10880 (2018).hotolyzed 1,3-diiodo-but-2-ene [(CH₂I)HC=C(CH₃)I] in the presence of O₂ with UV light and identified the Criegee intermediate syn-trans-MVKO as the main reaction product. However, the detailed mechanism for the production of MVKO is unexplored. It was assumed that photolysis of (CH₂I)HC=C(CH₃)I at 248 nm results in preferential dissociation of the weaker allylic C₍₁₎-I bond, rather than the vinylic (sp²-hybridized) C₍₃₎-I bond. Addition of O₂ to the C₍₃₎ atom, followed by breaking the C₍₃₎-I bond, produces the Criegee intermediate MVKO.
In this work we took the advantege of the diminished cage effect of solid para-hydrogen (p-H₂) as a matrix host to study the UV photodissociation of (Z)-(CH₂I)HC=C(CH₃)I. We report the formation and infrared identification of (Z)-3-iodo-but-2-en-1-yl [•CH₂CHC(CH₃)I] radical intermediate upon photodissociation of (Z)-(CH₂I)HC=C(CH₃)I in solid p-H₂ at 3.3 K with light at 280 nm. Lines at 3115.6, 2025.2, 3001.2, 2933.2, 2880.3, 2835.8, 1474.9, 1409.6, 1406.7, 1375.5, 1265.3, 1061.8, 1018.5, 1008.6, 922.0, 913.7, and 792.5 cm⁻¹ are assigned to (Z)-•CH₂CHC(CH₃)I. The assignments were derived according to behavior on secondary photolysis and comparison of the vibrational wavenumbers and the IR intensities of the observed lines with values predicted with the B2PLYP-D3/cc-pVTZ-pp method. No evidence of breakage of the C₍₃₎-I bond to form •C(CH₃)=C(CH₂I)H was observed.

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Footnotes:

V. P. Barber et al. J. Am. Chem. Soc., 140, 10866-10880 (2018).p

The 600-515 nm electronic band system of the jet-cooled SiCCl free radical has been studied by laser-induced fluorescence and single vibronic level emission spectroscopy. The radical was produced in an electric discharge through a dilute mixture of 1,1-dichlorosilacyclobutane in high pressure argon. The low-resolution LIF spectrum exhibits bands involving all three excited state vibrations, establishing values for the upper state vibrational frequencies. Emission spectra from thirteen upper state levels yielded the ground state bending and stretching energy levels up to 5000 cm⁻¹. These were satisfactorily fitted to a Renner-Teller model that included spin-orbit and vibrational anharmonicity effects. A high-resolution rotationally resolved spectrum of the ²Π_3/2 spin-orbit component of the 0-0 band was recorded and rotational analysis yielded accurate B values for both states of SiC³⁵Cl and SiC³⁷Cl. These constants were used with fixed ab initio C-Cl bond lengths to obtain r^" = 1.692(1) Å and r^′ = 1.594(1) Å. The bond lengths correspond to a silicon-carbon double bond in the ground state and an unusual Si-C triple bond in the excited state, similar to our previous findings for SiCH and SiCF. Halogenation has little effect on the Si-C bond length in both states.

We present first results from high-resolution infrared study of jet-cooled singlet monofluorocarbene (HCF) in the CH stretch region near 2600 cm⁻¹. Fully rotationally resolved absorption spectra of the CH stretch (ν₁) fundamental band, the ν₁+ν₃ ← ν₃ hot band, and part of the ν₂+ν₃ combination band are observed and analyzed, representing first experimental determination of rovibrational constants for these upper/lower levels. Transitions accessing each of the ν₁ K_a′ = 0, 1 and 2 manifolds are observed, with clear evidence for strong rovibrational coupling between the ν₁ K_a′ = 2 and ν₂+ν₃ K_a′ = 3 manifolds and allowing for perturbation analysis of a c-type Coriolis interaction between these two levels. The inclusion of such Coriolis coupling explains the large perturbations in both line positions and intensities observed for K_a′ = 2 and 3 subbands of the ν₁ and ν₂+ν₃ rovibrational bands, respectively. The results are in good agreement with, and also significantly extend, the analyses from previous studies of HCF using stimulated emission pumping from the first electronically excited state.

With five electronic states within 2 eV, the nitrate radical (NO₃) has a rich vibronic landscape for which photoelectron spectroscopy is an ideal probe. Here, we use slow photoelectron velocity map imaging of cryogenically cooled anions (cryo-SEVI), a high-resolution variant of anion photoelectron spectroscopy, to investigate the vibronic structure of the X̃²A′₂ state of NO₃. Our cryo-SEVI spectra are in excellent agreement with Franck-Condon simulations produced using a three-state Köppel-Domke-Cederbaum (KDC) Hamiltonian constructed for the NO₃ radical. Together, the experimental and simulated spectra provide clear evidence that the ν₃ fundamental resides near 1060 cm⁻¹, resolving a long-standing controversy surrounding this vibrational fundamental. Further, the appearance of activity along the ν₄ mode in this cryogenically-cold system verifies its activity through a Herzberg-Teller interaction, rather than as a hot band as previously suggested.

The X̃ ²A₂′ state of NO₃ under jet cooled conditions is investigated via laser induced fluorescence ( LIF ) and two-color resonant four-wave mixing ( 2C-R4WM ) techniques. The electronic structure of NO₃ is thought to be similar to that of BF₃, and the latter has been well documented in the literaturesH. B. Gray, Electrons and Chemical Bonding, W. A. Benjamin Inc., New York (1965); Open Source Tex Books, https://archive.org/details/ost-chemistry-electrons_chemical_bonding (retrieved Feb. 26, 2019).F. A. Cotton, Chemical Applications of Group Theory, 2nd ed., Wiley-International, New York (1971).. The BF₃ highest occupied molecular orbital ( HOMO ) possesses peculiar electronic structure with orbital localization on each of three F’s and no contribution on the center atom, B. For NO₃, the HOMO corresponds to a singly occupied molecular orbital ( SOMO ), and, in the X̃ ²A₂′ state ( Ã ²E" and B̃ ²E′, too ) of NO₃, the un-paired electron is localized on the three O’s and has no contribution on N. For this state, the degenerate vibrations are naturally expected to strongly affect the electron motion, which can be referred to as ”degenerate-vibrationally induced vibronic coupling” on the non-degenerate electronic state. The SOMO characteristics of NO₃ have been confirmed by high-level quantum chemical computationW. Eisfeld and K. Morokuma, J. Chem. Phys. 113, 5587 (2000). The characteristic features of the vibrational structure of the X̃ ²A₂′ state may possibly be understood by the vibronic coupling. One feature is an unexpectedly large spin splitting of 1₀ ( = N_K ) of the 3ν₄ (a₁′) level observed by 2C-R4WMM. Fukushima and T. Ishiwata, 73rd ISMS, paper WD02 (2018). and this splitting can be understood as the good quantum number behavior of P (  = K_v + Σ = Λ + l + Σ ) derived from the coupling.

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Footnotes:

H. B. Gray, Electrons and Chemical Bonding, W. A. Benjamin Inc., New York (1965); Open Source Tex Books, https://archive.org/details/ost-chemistry-electrons_chemical_bonding (retrieved Feb. 26, 2019).

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Footnotes:

W. Eisfeld and K. Morokuma, J. Chem. Phys. 113, 5587 (2000).. M. Fukushima and T. Ishiwata, 73rd ISMS, paper WD02 (2018).,

Recent transient Raman studies of the CO₂⁻ radical in water have led to the vibrational properties of this important radical intermediate. More importantly the evidence of the electron being shared between the bent CO₂ and its hydration shell at an energy of about ~ 0.28 eV above the ground electronic state is obtained. The corresponding electronic state has a life-time of several femtoseconds, consistent with the theoretical value. No evidence of non-equivalence of the two CO bonds was found, which suggested that the partially detached electron is symmetrically situated between CO₂ and the water molecule. Because of the electron not being localized on CO₂, the Raman scattered photon does not terminate into a vibrational state corresponding to the CO₂⁻ overtones above 0.28+/-0.03 eV or higher.¹ Now in a follow up study, we have examined its sulfur analogue CS₂⁻. We have prepared the radical anion in water by pulse radiolysis with Raman detection up to 4000 cm⁻¹to monitor its vibrational fingerprints. The Raman spectrum, excited in the resonance with the 270 nm (λ_max) absorption of CS₂⁻ is dominated by a very strong band at 666 cm⁻¹, associated with the symmetric C-S stretching vibration, its overtones, and combinations with SCS bending vibration of 330 cm⁻¹. Solvation shell bending and stretching modes are also enhanced, suggesting contribution to the excited state of the radical anion analogous to CO₂⁻. From the progression of overtones, and the first order anharmonicity of 2.57cm⁻¹, we estimate a continuum of vibrational states at an energy of roughly 5.4 eV. Unlike CO₂⁻, CS₂⁻ did not show any evidence of electron detachment up to the energies of  0.5eV. DFT calculations reproduced experimental frequencies fairly well predicting a molecular geometry of CS₂⁻ with CS bond lengths of 1.638Å and CSC angle of 143.3^o. (1) Janik I.,Tripathi G.N.R.; The nature of the CO₂⁻ radical anion in water. J. Chem. Phys. 2016, 144, 154307.

New tools for cooling and internal state control of molecules emerged in the past decade that allowed fast progress in the field of ultracold molecules. In this work, control of the rotational state of trapped ultracold SiO⁺ was utilized to study its spectroscopy and reaction dynamics. SiO⁺ was prepared by laser ablation of SiO followed by 1+1 REMPI. The ions were trapped in an RF Paul trap and sympathetically cooled by co-trapped ultracold atomic Ba⁺ ions. Rotational states of SiO⁺ were cooled and controlled by means of optical pumping with a spectrally pulse-shaped broadband light source. The pumping via the B²Σ-X²Σ electronic transition on rotational lines with selected N and ∆N was used to cool SiO⁺ molecules to the ground rotational state and to create a narrow distribution of occupied rotational states centered on a targeted level. Populations centered at different rotational states ranging from N=3 to N=65 were created. SiO⁺ quantum states were probed with photodissociation spectroscopy via the C²Π-X²Σ electronic transition. The C²Π state was experimentally characterized for the first time. Rotational state control was used to facilitate line assignment and to probe high rotational levels of the X²Σ state unpopulated at room temperature. Reaction of trapped SiO⁺ with H₂ was studied. The reaction rate of rotationally “thermal” SiO⁺ was found to be in good agreement with the literature value. The reaction rates of thermal H₂ with ultracold rotational state controlled SiO⁺ were measured. Reaction of H₂ with SiO⁺ “super rotor” states in which rotational energy is of the same order as bond dissociation energy and rotational period approaches time of collision was studied.

Yttrium (II) oxide (YO) is one of the first molecules that have been laser-cooled. Although the laser-cooling cycle in the reported experiment M. T. Hummon, M. Yeo, B. K. Stuhl, A. L. Collopy, Y. Xia, and J. Ye, Phys. Rev. Lett. 110, 143001 (2013) nvolves the à ² Π_1/2 − X̃ ² Σ⁺ (0, 0) transition, "dark’’ electronic states such as Ã'² ∆_3/2 also play important roles and directly affect the cooling efficiency. Moreover, the forbidden Ã'² ∆_3/2 − X̃ ² Σ⁺ transition can be utilized for further cooling of YO molecules. A. L Collopy, M. T Hummon, M. Yeo, B. Yan, and Jun Ye, New J. Phys. 17, 055008 (2015) o better understand the ro-vibronic structure of YO, we have obtained and analyzed the cavity ring-down (CRD) spectrum of the à ² Π_{3/2, 1/2} − X̃ ² Σ⁺ (0, 0) transition of jet-cooled YO molecules. Detection of the Ã'² ∆− X̃ ² Σ⁺ (0, 0) transition is in process. We will discuss experimental measures that will be taken to further improve the signal-to-noise ratio for the pursuit of “dark”-state spectra of YO and other candidate molecules for laser cooling. M. T. Hummon, M. Yeo, B. K. Stuhl, A. L. Collopy, Y. Xia, and J. Ye, Phys. Rev. Lett. 110, 143001 (2013) i A. L Collopy, M. T Hummon, M. Yeo, B. Yan, and Jun Ye, New J. Phys. 17, 055008 (2015) T