Spin-rotation Splittings Research Articles

Spectroscopy and ion thermometry of C2− using laser-cooling transitions

A prerequisite for laser cooling a molecular anion, which has not been achieved so far, is the precise knowledge of the relevant transition frequencies in the cooling scheme. To determine these frequencies we present a versatile method that uses one pump and one photodetachment light beam. We apply this approach to C$_{2}^{-}$ and study the laser cooling transitions between the electronic ground state and the second electronic excited state in their respective vibrational ground levels, $B ^{2} \Sigma _{u} ^{+}(v=0) \leftarrow X ^{2} \Sigma _{g} ^{+}(v=0) $. Measurements of the R(0), R(2), and P(2) transitions are presented, which determine the transition frequencies with a wavemeter-based accuracy of $0.7\times10^{-3}$ cm$^{-1}$ or 20 MHz. The spin-rotation splitting is resolved, which allows for a more precise determination of the splitting constants to $\gamma ' = 7.15(19)\times10^{-3}$ cm$^{-1}$ and $\gamma '' = 4.10(27)\times10^{-3}$ cm$^{-1}$. These results are used to characterize the ions in the cryogenic 16-pole wire trap employed in this experiment. The translational and rotational temperature of the ions cooled by helium buffer gas are derived from the Doppler widths and the amplitude ratios of the measured transitions. The results support the common observation that the translational temperature is higher than the buffer gas temperature due to collisional heating under micromotion, in particular at low temperatures. Additionally, a rotational temperature significantly lower than the translational is measured, which agrees with the notion that the mass weighted collision temperature of the C$_{2}^{-}$-He system defines the internal rotational state population.

Interaction of the HCO radical with molecular hydrogen: Ab initio potential energy surface and scattering calculations.

The potential energy surface describing the interaction of the HCO radical with molecular hydrogen has been computed through explicitly correlated coupled cluster calculations including single, double, and (perturbative) triple excitations [RCCSD(T)-F12a], with the assumption of fixed molecular geometries. The computed points were fit to an analytical form suitable for time-independent quantum scattering calculations of rotationally inelastic cross sections and rate coefficients. Since the spin-rotation splittings in HCO are small, cross sections for fine-structure resolved transitions are computed with electron-spin free T matrix elements through the recoupling technique usually employed to determine hyperfine-resolved cross sections. Both spin-free and fine-structure resolved state-to-state cross sections for rotationally inelastic transitions are presented and discussed.

The electronic spectrum of the jet-cooled stibino (SbH2) free radical.

The Ã2A1-X̃2B1 electronic transition of the jet-cooled stibino (SbH2 and SbD2) free radical has been observed for the first time using laser induced fluorescence (LIF) detection. The radicals were produced by a pulsed electric discharge through a mixture of stibine (SbH3 or SbD3) in high pressure argon at the exit of a pulsed molecular beam valve. SbH2 exhibits only three LIF bands, assigned as 21 0, 00 0, and 20 1, with a fluorescence lifetime (τ), which decreases from ∼50 ns for 00 to <10 ns for 21. LIF transitions to the 00 (τ ∼ 2 µs), 21 (τ ∼ 400 ns), and 22 (τ ∼ 75 ns) upper vibronic states of SbD2 were also observed. High-resolution spectra exhibited large spin-rotation splittings and small resolved antimony hyperfine splittings due to a substantial Fermi contact interaction in the excited state. The experimentally determined rotational constants gave effective molecular structures of r0 ″ = 1.724(2) Å, θ0 ″ = 90.38(7)° and r0 ' = 1.693(6) Å, θ0 ' = 120.6(3)°. The ground state bending vibrational levels up to eight quanta (6404 cm-1) in SbH2 and 12 quanta (6853 cm-1) in SbD2 were measured from dispersed fluorescence spectra. All indications are that SbH2 undergoes a dissociative process at low vibrational energies in the excited electronic state.

The missing halide: Millimeter-wave spectroscopy of the MgI radical (X2Σ+)

The pure rotational spectrum of the MgI radical in its ground electronic state (X2Σ+) has been measured using millimeter/submillimeter wave direct absorption techniques in the region of 200–300GHz. The molecule was created in a DC discharge by the reaction of magnesium vapor, produced in a Broida-type oven, with CH3I. Between five to twelve transitions were recorded for the v=0, 1, and 2 vibrational states of 24MgI, as well as the v=0 state for the isotopologues 25MgI and 26MgI, measured in their natural magnesium abundance. All observed transitions exhibited large spin-rotation splittings. Rotational, centrifugal distortion, and spin-rotation constants were determined for each isotopologue and the excited vibrational states of 24MgI. Equilibrium parameters Be, αe, De, βe, γe and γe′ were derived for the main isotopologue, as well as the equilibrium bond length, re=2.5730 (1)Å. The large spin-rotation constant of γ∼300MHz in MgI is thought to arise from second-order spin-orbit coupling originating in the nearby A2Π state, with possible contributions from other excited 2Π states.

Sub-Doppler infrared spectroscopy of CH2OH radical in a slit supersonic jet: Vibration-rotation-tunneling dynamics in the symmetric CH stretch manifold.

The sub-Doppler CH-symmetric stretch (ν3) infrared absorption spectrum of a hydroxymethyl (CH2OH) radical is observed and analyzed with the radical formed in a slit-jet supersonic discharge expansion (Trot = 18 K) via Cl atom mediated H atom abstraction from methanol. The high sensitivity of the spectrometer and reduced spectral congestion associated with the cooled expansion enable first infrared spectroscopic observation of hydroxymethyl transitions from both ± symmetry tunneling states resulting from large amplitude COH torsional motion. Nuclear spin statistics due to exchange of the two methyl H-atoms aid in unambiguous rovibrational assignment of two A-type Ka = 0 ← 0 and Ka = 1 ← 1 bands out of each ± tunneling state, with additional spectral information obtained from spin-rotation splittings in P, Q, and R branch Ka = 1 ← 1 transitions that become resolved at low N. A high level ab initio potential surface (CCSD(T)-f12b/cc-pvnzf12 (n = 2,3)/CBS) is calculated in the large amplitude COH torsional and CH2 wag coordinates, which in the adiabatic approximation and with zero point correction predicts ground state tunneling splittings in good qualitative agreement with experiment. Of particular astrochemical interest, a combined fit of the present infrared ground state combination differences with recently reported millimeter-wave frequencies permits the determination of improved accuracy rotational constants for the ground vibrational state, which will facilitate ongoing millimeter/microwave searches for a hydroxymethyl radical in the interstellar medium.

The pure rotational spectrum of the ScO (X2Σ+) radical

The rotational spectrum of ScO (X2Σ+) has been measured in the gas phase in the frequency range 30–493GHz using a combination of Fourier transform microwave/millimeter-wave (FTM/mmW) and submillimeter direct absorption methods. This work is the first pure rotational study of this radical. Both the ground vibrational and v=1 states were observed. ScO was created from the reaction of metal vapor, produced either by a laser ablation source or a Broida-type oven, and N2O, in the former case heavily diluted in argon. Extensive hyperfine structure was observed in the FTM/mmW data, although the spin-rotation splitting was found to be small (∼3MHz). In the mm-wave spectra, however, the fine and hyperfine structure was blended together, resulting in broad, single lines for a given transition N+1←N. The data were analyzed in a combined fit using the very accurate hyperfine measurements of Childs and Steimle (1988), employing a Hund’s case b Hamiltonian, and an improved set of rotational and centrifugal distortion constants were determined. These measurements improve the accuracy of predicted frequencies for astronomical searches by 14–18MHz, or 16–20km/s, in the 1mm region – a difference of half to a full linewidth for certain interstellar sources. This work also demonstrates the capabilities of the FTM/mmW spectrometer at 61GHz.

High-resolution laser spectroscopy and magnetic effect of the $\tilde B$B̃2E′ ← $\tilde X$X̃2A2′ transition of 14NO3 radical

Rotationally resolved high-resolution fluorescence excitation spectra of (14)NO3 radical have been observed for the 662 nm band, which is assigned as the 0-0 band of the B̃(2)E' ←X̃(2)A2' transition, by crossing a single-mode laser beam perpendicularly to a collimated molecular beam. More than 3000 rotational lines were detected in 15,070-15,145 cm(-1) region, but it is difficult to find the rotational line series. Remarkable rotational line pairs, whose interval is about 0.0246 cm(-1), were found in the observed spectrum. This interval is the same amount with the spin-rotation splitting of the X̃(2)A2' (υ = 0, k = 0, N = 1) level. From this interval and the observed Zeeman splitting up to 360 G, seven line pairs were assigned as the transitions to the (2)E'(3/2) (J' = 1.5) levels and 15 line pairs were assigned as the transitions to the (2)E'(1/2) (J' = 0.5) levels. From the rotational analysis, we recognized that the (2)E' state splits into (2)E'(3/2) and (2)E'(1/2) by the spin-orbit interaction and the effective spin-orbit interaction constant was roughly estimated as -21 cm(-1). From the number of the rotational line pairs, we concluded that the complicated rotational structure of this 662 nm band of (14)NO3 mainly owes to the vibronic interaction between the B̃(2)E' state and the dark Ã(2)E″ state through the a2″ symmetry vibrational mode.

Rotational Spectroscopy of Isotopologues of Silicon Monoxide, SiO, and Spectroscopic Parameters from a Combined Fit of Rotational and Rovibrational Data

Pure rotational transitions of silicon monoxide, involving the main ((28)Si(16)O) as well as several rare isotopic species, were observed in their ground vibrational states by employing long-path absorption spectroscopy between 86 and 825 GHz (1 ≤ J" ≤ 18). Fourier transform microwave spectroscopy was used to study the J" = 0 transition frequencies in the ground and several vibrationally excited states. The vibrational excitation of the newly studied isotopologues extend to between υ = 9 and 29 for (28)Si(17)O and (30)Si(16)O, respectively. Data were extended for some previously investigated species up to υ = 51 for the main isotopologue. The high spectral resolution allowed us to resolve the hyperfine structure in (28)Si(17)O caused by the nuclear electric quadrupole and magnetic dipole moments of (17)O for the first time, and to resolve the much smaller nuclear spin-rotation splitting for isotopic species containing (29)Si. These data were combined with previous rotational and rovibrational (infrared) data to determine an improved set of spectroscopic parameters of SiO in one global fit which takes the breakdown of the Born-Oppenheimer approximation into account. Highly accurate rotational transition frequencies for this important astronomical molecule can now be predicted well into the terahertz region with this parameter set. In addition, a more complete comparison among physical properties of group 14/16 diatomics is possible.

Pulsed discharge jet electronic spectroscopy of the aluminum dicarbide (AlC2) free radical

Laser-induced fluorescence and wavelength resolved emission spectra of the C ̃(2)B(2)-X̃ (2)A(1) band system of the gas phase aluminum dicarbide free radical have been obtained using the pulsed discharge jet technique. The radical was produced by electron bombardment of a precursor mixture of trimethylaluminum in high-pressure argon. The three vibrational frequencies of T-shaped AlC(2) have been determined in both the combining states along with several of the anharmonicity constants. The 0(0)(0) band has been recorded with high resolution and rotationally analyzed. The spectrum is complicated by partially resolved spin-rotation and aluminum hyperfine splittings. Where necessary, we have fixed the spin-rotation constants used in the rotational analysis at the values predicted by density functional theory. The derived molecular structures are: r(0)('')(C-C) = 1.271(2) Å, r(0)('')(Al-C) = 1.926(1) Å, θ(")(C-Al-C) = 38.5(2)°, r(0)(')(C-C) = 1.323(2) Å, r(0)(')(Al-C) = 1.934(1) Å, and θ(')(C-Al-C) = 40.0(2)°. Unlike SiC(2), aluminum dicarbide shows no spectroscopic evidence of facile isomerization to the linear structure in the ground electronic state.

High-resolution laser spectroscopy of the [formula omitted] transition of MgC 4H

The 0 0 0 band of the A ˜ 2 Π - X ˜ 2 Σ + transition of MgC 4H has been recorded at high resolution using laser-induced fluorescence. The molecules were produced using an ablation source by the reaction of magnesium and ∼10% acetylene seeded in helium. The rotationally-resolved high-resolution data were fitted to obtain rotational and fine structure parameters for both states. The centrifugal distortion, upper state Λ-doubling and ground state spin-rotation splitting parameters were all found to be negligible and the reasons for this are investigated and discussed. The 3 0 1 band heads of the A ˜ 2 Π - X ˜ 2 Σ + transition of MgC 6H were obtained but the rotational structure was not resolved.

Isotopic study of the [formula omitted] transition of calcium monomethoxide using laser excitation and population depletion spectroscopy

High resolution excitation spectra have been obtained of the 0–0 band of the B ∼ 2 A 1 – X ∼ 2 A 1 transition of four isotopologues, CaO 12CH 3, CaO 13CH 3, CaO 12CD 3 and CaO 13CD 3 of calcium monomethoxide. The deuterated species were found to have unexpectedly complicated spectra, and definitive rotational assignments were possible only from investigation by optical optical double resonance (OODR) population depletion spectroscopy. This confirmed the assignment of the CaO 12CD 3 spectrum, and proved crucial in assigning the K-structure and spin components for CaO 13CD 3. The B ∼ 2 A 1 state was found to be well described by the symmetric rotor model with C 3 v symmetry for both hydride species but, for the deuterides, the K-structure and spin rotation splittings were irregular, especially for CaO 13CD 3 where the K = 0 and 1 levels were heavily perturbed. The changes in the A constant determined for the hydride suggest that the CH 3 umbrella opens by ∼0.4°, i.e., 0.2° further on excitation to the B ∼ 2 A 1 state than on excitation to the lower-lying A ∼ 2 E state (geometry change established in an earlier experiment by Crozet et al. [P. Crozet, A.J. Ross, C. Linton, A.G. Adam, W.S. Hopkins, R.J. Le Roy, J. Mol. Spectrosc. 229 (2005), 224–230]).

Rotational spectroscopy of the isotopic species of silicon monosulfide, SiS

Pure rotational transitions of silicon monosulfide ((28)Si(32)S) and its rare isotopic species have been observed in their ground as well as vibrationally excited states by employing Fourier transform microwave (FTMW) spectroscopy of a supersonic molecular beam at centimetre wavelengths (13-37 GHz) and by using long-path absorption spectroscopy at millimetre and submillimetre wavelengths (127-925 GHz). The latter measurements include 91 transition frequencies for (28)Si(32)S, (28)Si(33)S, (28)Si(34)S, (29)Si(32)S and (30)Si(32)S in upsilon = 0, as well as 5 lines for (28)Si(32)S in upsilon = 1, with rotational quantum numbers J''< or = 52. The centimetre-wave measurements include more than 300 newly recorded lines. Together with previous data they result in almost 600 transitions (J'' = 0 and 1) from all twelve possible isotopic species, including (29)Si(36)S and (30)Si(36)S, which have fractional abundances of about 7 x 10(-6) and 4.5 x 10(-6), respectively. Rotational transitions were observed from upsilon = 0 for the least abundant isotopic species to as high as upsilon = 51 for the main species. Owing to the high spectral resolution of the FTMW spectrometer, hyperfine structure from the nuclear electric quadrupole moment of (33)S was resolved for species containing this isotope, as was much smaller nuclear spin-rotation splitting for isotopic species involving (29)Si. By combining the measurements here with previously published microwave and infrared data in one global fit, an improved set of spectroscopic parameters for SiS has been derived which include several terms describing the breakdown of the Born-Oppenheimer approximation. With this parameter set, highly accurate rotational frequencies for this important astronomical molecule can now be predicted well into the terahertz region.

Direct deperturbation analysis of the AΠ2∼BΣ+2 complex of LiAr7,6 isotopomers

Direct deperturbation analysis of the highly accurate experimental rovibronic term values of the A (2)Pi approximately B (2)Sigma(+) complex of LiAr [R. Bruhl and D. Zimmermann, J. Chem. Phys. 114, 3035 (2001)] has been performed in the framework of inverted close-coupling approach implicitly adjusted to the unified treatment of the overall A approximately B coupling effect without reducing the rovibrational dimensionality. The nonlinear fitting procedure was supported by the ab initio calculations on the spin-orbit and angular coupling matrix elements between the lowest X (2)Sigma(+), A (2)Pi, and B (2)Sigma(+) states. The analytical grid mapping based on the reduced variable representation of the radial coordinate r was used to improve the efficiency of the solution of the close-coupling radial equations near the dissociation limit. The mutual A approximately X perturbation effect on the A (2)Pi term values and spin-rotation splitting of the ground state were evaluated for both (7,6)LiAr isotopomers. The resulting empirical potential-energy curves for the adiabatic A (2)Pi and B (2)Sigma(+) states, along with the refined r-dependent nonadiabatic matrix elements, reproduce the total rovibronic structure of the (7)LiAr complex with the standard deviation of 0.003 cm(-1). The mass invariance of the deperturbed electronic parameters was confirmed by the calculation of the rovibronic term values of the (6)LiAr isotopomer which coincided with their experimental counterparts within 0.004 cm(-1).

Hyperfine coupling and pseudorotational motion interaction in Na3

Hyperfine patterns calculations are carried out for the Na3 cluster with a view towards understanding the microwave measurements which were performed for three rotational transitions belonging to the ground X̃ electronic state. The calculations take simultaneously into account the pseudorotational motion, the spin-rotation coupling, and the magnetic electron spin-nuclear spin hyperfine coupling. Matching calculated and observed patterns suggests that the cluster is characterized by small pseudorotational tunneling and spin-rotation splittings, some amount of Fermi contact interaction at the two terminal nuclei, and significant dipolar spin-nuclear spin coupling for the central atom.

The pure rotational spectra of SrSH (X̃ 2A′) and SrS (X 1Σ+): Further studies in alkaline-earth bonding

The pure rotational spectrum of the SrSH radical in its ground electronic (X̃ 2A′) and vibrational states has been measured using millimeter/submillimeter-wave direct absorption techniques. This work is the first observation of SrSH with rotational resolution. The spectrum of its deuterium isotopomer SrSD and SrS (X 1Σ+) has been recorded as well. These species were created by the reaction of strontium vapor and H2S, in the presence of a dc discharge. SrS was also made with CS2. For SrSH and SrSD, eight rotational transitions were recorded, respectively, for which asymmetry components up to Ka=8 were measured; fine structure was also resolved in each component. Thirteen transitions of SrS in each of its v=0, 1, and 2 states have additionally been observed. These data have been analyzed and spectroscopic parameters determined for all three species, including spin-rotation terms for the strontium hydrosulfides. From an r0 structure calculation, the bond angle in SrSH was determined to be 91.48(3)°, very close to that of H2S and CaSH. This geometry indicates that SrSH is a covalently bonded molecule, as opposed to linear (and ionic) SrOH. The Sr–S bond length in SrSH was also found to be greater than that of SrS (rSr—S=2.705 Å versus 2.441 Å), indicating a change in bond order. In addition, the spin-rotation interaction in SrSH and SrSD includes a small contribution from the off-diagonal term, (εab+εba)/2, resulting from the crossing of energy levels with ΔJ=0, ΔKa=±1. Second-order spin-orbit coupling appears to make a significant contribution to the spin-rotation splitting, as well, which must arise from mixing of the à 2A′ and B̃ 2A″ excited states.

The visible laser excitation spectrum of YbOH: The à 2Π–X̃2Σ+ transition

The à 2Π–X̃ 2Σ+ electronic transition of YbOH has been analyzed for the first time. Gas-phase YbOH was produced in a Broida-type oven by the reaction of ytterbium metal vapor with H2O2 vapor. Low-resolution spectra recorded with a broadband dye laser were used to examine the vibrational structure of the ÖX̃ system while high-resolution laser spectroscopy with a single-frequency dye laser was employed to obtain spectra of the 000–000 and 000–100 bands of YbOH174 and the 000–000 band of YbOH172 (where v1 is the Yb–O stretching mode). The rotational analysis of the à 2Π–X̃ 2Σ+ transition demonstrates YbOH is a linear molecule, much like the alkaline–earth monohydroxides. Rotational and vibrational parameters are presented and the unusual behavior of the ground state spin-rotation splitting is discussed through a comparison with the isoelectronic molecule YbF.

Laboratory Detection of the M[CLC]g[/CLC]NH[TINF]2[/TINF] Radical ([FORMULA][F][A][AC]X[/AC][AC][d5][/AC][/A][HSP SP="0.167"][ZW][SUP]2[/SUP]A[INF]1[/INF][/F][/FORMULA

The MgNH2 radical has been detected in the laboratory for the first time in its 2A1 ground state using techniques of millimeter-wave spectroscopy. This short-lived species was created using Broida oven methods. Multiple Ka asymmetry components were recorded for MgNH2 in 12 rotational transitions in the frequency range 129-527 GHz, and spin-rotation splittings were resolved in every component. A total of 251 separate lines were measured. An intensive search for a possible inversion state in MgNH2 was also conducted but proved negative. This data set has been modeled with an S-reduced Hamiltonian and very accurate rotational, centrifugal distortion, and spin-rotation parameters have been determined for magnesium amide. This study strongly suggests that the radical is planar with C2v symmetry, although small barriers to planarity cannot be ruled out. MgNH2 could be produced in interstellar/circumstellar gas from the association reaction of Mg+ + NH3, as predicted by theory.

High-Resolution Laser Spectroscopy of the Ã2B2–X̃2A1 and B̃2B1–X̃2A1 Systems of SrNH2

The 000 bands of the Ã2B2–X̃2A1 and B̃2B1–X̃2A1 systems of SrNH2 were observed at Doppler-limited resolution using a Broida oven source and laser-induced fluorescence detection. A full rotational analysis of both transitions was performed including Ka levels up to 5 and J levels up to 55. The B̃2B1 state was found to be extensively perturbed and only some of the subbands could be analyzed. The Ã2B2 and B̃2B1 states undergo a strong Coriolis-type interaction which results in extremely large spin–rotation splittings in both states, effectively splitting all levels with Ka′ ≠ 0 into two well-separated spin components.

High-Resolution Laser Spectroscopy of YbCl: The A2Π–X2Σ+ Transition

The A2Π–X2Σ+ transition of 174Yb35Cl and 172Yb35Cl has been rotationally analyzed for the first time. Doppler-limited laser excitation spectroscopy with selective detection of fluorescence was used to obtain spectra of the 0–0 and 1–0 bands with a measurement accuracy of approximately 0.0035 cm−1. Resolved fluorescence was used to record the 0–1, 0–2, and 0–3 bands and to unequivocally assign the rotational numbering, N, to the laser excitation spectra. In total, over 1300 line positions have been measured and assigned for each of the two isotopomers and employed in least-squares fits of molecular parameters. The principal results for the A2Π state are Ae = 1491.494(2) cm−1 and Re = 2.4433(1) Å, and for the X2Σ+ state, Re = 2.4883(2) Å and γe = 4.59(2) × 10−3 cm−1. The interaction between the X2Σ+ and A2Π states has been investigated and is shown to be the main contributor to the spin–rotation splitting in the ground state.

The microwave spectrum of the cesium monoxide CsO radical

The microwave spectrum of CsO has been observed and analyzed, not only in the ground vibrational state, but also in the v=1–3 excited vibrational states. The CsO radical was generated by the reaction of N2O with Cs vapor, which was produced by the reaction of Li metal with CsCl at 500–530 °C. The observed spectra were found to conform to those expected for a Σ2 diatomic molecule, thereby establishing the ground electronic state of CsO to be of Σ2. The observed rotational and centrifugal distortion constants yielded the equilibrium bond length and the harmonic vibrational frequency to be 2.300 745 (16) Å and 356.78 (11) cm−1, respectively, based on the Born–Oppenheimer approximation. A careful examination of the observed spectral pattern definitely concluded that the spin-rotation interaction constant was positive, at variance with the expectation from a simple Σ2/2Π two-states interaction. This observation was interpreted by assuming positive contributions from higher excited electronic states which superseded a negative contribution from the Π2 lowest excited state; the latter state was responsible for the large dependence of the spin-rotation interaction constant on the vibrational quantum number and was estimated from this vibrational dependence to be located at 1225 cm−1 above the ground electronic state. In reverse to the spin-rotation splitting, the hyperfine splitting was found to increase with the vibrational excitation; in the v=3 state the hyperfine structure was found completely resolved. However, the hyperfine coupling constants did not vary much with the vibrational quantum number, namely the vibrational dependence of the hyperfine splitting was caused primarily by that of the spin-rotation splitting. The observed hyperfine interaction constants indicated that CsO was an ionic molecule.