Nuclear Spin Modifications Research Articles

Rotational excitation of methanol in collisions with molecular hydrogen

ABSTRACT This paper presents rate coefficients for transitions between rotational levels of the A-type and E-type nuclear spin modifications of methanol induced by collisions with molecular hydrogen. These rate coefficients are required for an accurate determination of methanol abundance in the interstellar medium, where local thermodynamic equilibrium conditions generally do not apply. Time-independent close-coupling quantum scattering calculations have been employed to calculate cross-sections and rate coefficients for the (de-)excitation of methanol in collisions with para- and ortho-H2. These calculations utilized a potential energy surface (PES) for the interaction of methanol with H2 recently computed by the explicitly correlated CCSD(T)-F12a coupled-cluster method that employed a correlation-consistent aug-cc-pVTZ basis. Rate coefficients for temperatures ranging from 3 to 250 K were calculated for all transitions among the first 76 rotational levels of both A-type and E-type methanol, whose energies are less than or equal to 170 K. These rate coefficients are compared with those by Rabli and Flower who carried out coupled-state calculations using a PES computed by second-order many-body perturbation theory. Simple radiative transfer calculations using the present set of rate coefficients are also reported and compared with such calculations using the rate coefficients previously computed by Rabli and Flower.

Fast ortho-to-para conversion of molecular hydrogen in chemisorption and matrix-isolation systems.

Molecular hydrogen has two nuclear-spin modifications called ortho and para. Because of the symmetry restriction with respect to permutation of the two protons, the ortho and para isomers take only odd and even values of the rotational quantum number, respectively. The ortho-to-para conversion is promoted in condensed systems, to which the excess rotational energy and spin angular momentum are transferred. We review recent studies on fast ortho-to-para conversion of hydrogen in molecular chemisorption and matrix isolation systems, discussing the conversion mechanism as well as rotational-relaxation pathways.

Collisional excitation of the formyl radical (HCO) by molecular hydrogen

ABSTRACT This paper addresses the need for accurate rate coefficients for transitions between fine- and hyperfine-structure resolved rotational transitions in the formyl (HCO) radical induced by collisions with the two nuclear spin modifications of H2, the dominant molecule in the interstellar medium (ISM). These rate coefficients, as well as radiative transition rate coefficients, are required for accurate determination of the abundance of HCO in the ISM. Time-independent close-coupling quantum scattering calculations have been used to compute rate coefficients for (de-)excitation of HCO in collisions with para- and ortho-H2. These calculations utilized a potential energy surface for the interaction of HCO with H2 recently computed by the explicitly correlated RCCSD(T)-F12a coupled-cluster method. Rate coefficients for temperatures ranging from 5 to 400 K were calculated for all transitions among the fine and hyperfine levels associated with the first 22 rotational levels of HCO, whose energies are less than or equal to 144 K.

Ab initio potential and rotational spectra of the CO-N2 complex.

Ab initio calculations of the intermolecular potential energy surface (PES) of CO-N2 have been carried out using the closed-shell single- and double-excitation coupled cluster approach with a non-iterative perturbative treatment of triple excitations method and the augmented correlation-consistent quadruple-zeta (aug-cc-pVQZ) basis set supplemented with midbond functions. The global minimum (De = 117.35 cm-1) of the four-dimensional PES corresponds to an approximately T-shaped structure with the N2 subunit forming the leg and CO the top. The bound rovibrational levels of the CO-N2 complex were calculated for total angular momenta J = 0-8 on this intermolecular potential surface. The calculated dissociation energies D0 are 75.60 and 76.79 cm-1 for the ortho-N2 (A-symmetry) and para-N2 (B-symmetry) nuclear spin modifications of CO-N2, respectively. Guided by these bound state calculations, a new millimeter-wave survey for the CO-N2 complex in the frequency range of 110-145 GHz was performed using the intracavity OROTRON jet spectrometer. Transitions not previously observed were detected and assigned to the subbands connecting the K = 0 and 1, (jCO, jN2 ) = (1, 0) states with a new K = 1, (jCO, jN2 ) = (2, 0) state. Finally, the measured rotational energy levels of the CO-N2 complex were compared to the theoretical bound state results, thus providing a critical test of the quality of the PES presented. The computed rovibrational wave functions were analyzed to characterize the nature of the different bound states observed for the two nuclear spin species of CO-N2.

Rotational study of the CH4-CO complex: Millimeter-wave measurements and ab initio calculations.

The rotational spectrum of the van der Waals complex CH4-CO has been measured with the intracavity OROTRON jet spectrometer in the frequency range of 110-145 GHz. Newly observed and assigned transitions belong to the K = 2-1 subband correlating with the rotationless jCH4 = 0 ground state and the K = 2-1 and K = 0-1 subbands correlating with the jCH4 = 2 excited state of free methane. The (approximate) quantum number K is the projection of the total angular momentum J on the intermolecular axis. The new data were analyzed together with the known millimeter-wave and microwave transitions in order to determine the molecular parameters of the CH4-CO complex. Accompanying ab initio calculations of the intermolecular potential energy surface (PES) of CH4-CO have been carried out at the explicitly correlated coupled cluster level of theory with single, double, and perturbative triple excitations [CCSD(T)-F12a] and an augmented correlation-consistent triple zeta (aVTZ) basis set. The global minimum of the five-dimensional PES corresponds to an approximately T-shaped structure with the CH4 face closest to the CO subunit and binding energy De = 177.82 cm(-1). The bound rovibrational levels of the CH4-CO complex were calculated for total angular momentum J = 0-6 on this intermolecular potential surface and compared with the experimental results. The calculated dissociation energies D0 are 91.32, 94.46, and 104.21 cm(-1) for A (jCH4 = 0), F (jCH4 = 1), and E (jCH4 = 2) nuclear spin modifications of CH4-CO, respectively.

Effects of rotational-symmetry breaking on physisorption of ortho- and para-H2 on Ag(111).

Quantum-state-selective thermal desorption of H2 weakly physisorbed on Ag(111) demonstrates significantly different desorption features between the nuclear-spin modifications. An energy shift due to the rotational-symmetry breaking induced by an anisotropic interaction affects not only the enthalpy but also the entropy of adsorption. The preexponential factor for desorption of the ortho-H2 is about three times as large as that of the para-H2. The entropy difference indicates a perpendicular orientation preference of anisotropic physisorption potential, which also suggests the importance of partial hybridization interaction for weak physisorption.

Nuclear spin conversion in diatomic molecules

A mechanism of the internal interaction in dimers that mixes different nuclear spin modifications has been proposed. It has been shown that the intramolecular current associated with transitions between electronic terms of different parities can generate different magnetic fields on nuclei, leading to transitions between spin modifications and to the corresponding changes in rotational states. In the framework of the known quantum relaxation process, this interaction initiates irreversible conversion of nuclear spin modifications. The estimated conversion rate for nitrogen at atmospheric pressure is quite high (10−3–10−5 s−1).

On the Symmetry and Degeneracy of H3+

The fundamental molecular ion H3(+) has impacted astronomy, chemistry, and physics, particularly since the discovery of its rovibrational spectrum. Consisting of three identical fermions, its properties are profoundly influenced by the requirements of exchange symmetry, most notably the nonexistence of its ground rotational state. Spectroscopy of H3(+) is often used to infer the relative abundances of its two nuclear spin modifications, ortho- and para-H3(+), which are important in areas as diverse as electron dissociative recombination and deuterium fractionation in cold interstellar clouds. In this paper, we explore in detail the impact of exchange symmetry on the states of H3(+), with a particular focus on the state degeneracies necessary for converting spectral transition intensities to relative abundances. We address points of confusion in the literature surrounding these issues and discuss the implications for proton-transfer reactions of H3(+) at low temperatures.

Overtone vibrational spectroscopy in H2-H2O complexes: A combined high level theoretical ab initio, dynamical and experimental study

First results are reported on overtone (v(OH) = 2 ← 0) spectroscopy of weakly bound H(2)-H(2)O complexes in a slit supersonic jet, based on a novel combination of (i) vibrationally mediated predissociation of H(2)-H(2)O, followed by (ii) UV photodissociation of the resulting H(2)O, and (iii) UV laser induced fluorescence on the nascent OH radical. In addition, intermolecular dynamical calculations are performed in full 5D on the recent ab initio intermolecular potential of Valiron et al. [J. Chem. Phys. 129, 134306 (2008)] in order to further elucidate the identity of the infrared transitions detected. Excellent agreement is achieved between experimental and theoretical spectral predictions for the most strongly bound van der Waals complex consisting of ortho (I = 1) H(2) and ortho (I = 1) H(2)O (oH(2)-oH(2)O). Specifically, two distinct bands are seen in the oH(2)-oH(2)O spectrum, corresponding to internal rotor states in the upper vibrational manifold of Σ and Π rotational character. However, none of the three other possible nuclear spin modifications (pH(2)-oH(2)O, pH(2)-pH(2)O, or oH(2)-pH(2)O) are observed above current signal to noise level, which for the pH(2) complexes is argued to arise from displacement by oH(2) in the expansion mixture to preferentially form the more strongly bound species. Direct measurement of oH(2)-oH(2)O vibrational predissociation in the time domain reveals lifetimes of 15(2) ns and <5(2) ns for the Σ and Π states, respectively. Theoretical calculations permit the results to be interpreted in terms of near resonant energy levels and intermolecular alignment of the H(2) and H(2)O wavefunctions, providing insight into predissociation dynamical pathways from these metastable levels.

Theoretical investigation of rotationally inelastic collisions of the methyl radical with helium

Rotationally inelastic collisions of the CH(3) molecule in its ground X(2)A(2)'' electronic state have been investigated. We have determined a potential energy surface (PES) for the interaction of rigid CH(3), frozen at its equilibrium geometry, with a helium atom, using a coupled-cluster method that includes all single and double excitations, as well as perturbative contributions of connected triple excitations [RCCSD(T)]. The anisotropy of the PES is dominated by repulsion of the helium by the hydrogen atoms. The dissociation energy D(e) was computed to equal 27.0 cm(-1). At the global minimum, the helium atom lies in the CH(3) plane between two C-H bonds at an atom-molecule separation R = 6.52 bohr. Cross sections for collision-induced rotational transitions have been determined through quantum scattering calculations for both nuclear spin modifications. Rotationally inelastic collisions can cause a change in the rotational angular momentum n and its body-frame projection k. Because of the anisotropy of the PES due to the hydrogen atoms, there is a strong propensity for Δk = ±3 transitions. Thermal rate constants for state-specific total collisional removal have also been determined.

Unveiling the nonadiabatic rotational excitation process in a symmetric-top molecule induced by two intense laser pulses

We experimentally investigate the nonadiabatic rotational excitation process of a symmetric-top molecule, benzene, in the electronic ground state irradiated by intense nonresonant ultrafast laser fields. The initial rotational-state distribution was restricted mostly to the five lowest levels with different nuclear spin modifications by an extensive adiabatic cooling with the rotational temperature well below 1 K, and distributions after the interaction with a femtosecond double-pulse pair (3-5 TW/cm(2) each with 160 fs duration) with time delays were probed in a quantum-state resolved manner by employing resonant enhanced multiphoton ionization via the S(1) ← S(0) 6(0) (1) vibronic transition. Populations of 10 rotational levels with J ranging from 0 to 4 and K from 0 to 3 were examined to show an oscillatory dependence on the time delay between the two pulses. Fourier analysis of the beat signals provides the coupling strengths between the constituent levels of the rotational wave packets created by the nonadiabatic excitation. These data are in good agreement with the results from quantum mechanical calculations, evidencing stepwise excitation pathways in the wave packet creation with ΔJ = 2 in the K = 0 stack while ΔJ = 1 and 2 in the K > 0 stacks.

Electric-field-induced nuclear-spin flips mediated by enhanced spin–orbit coupling

Molecular hydrogen exists in two forms, which differ in the relative orientation of their nuclear spins. Interconversion between the two isomeric forms is extremely rare, unless there is an interaction breaking the symmetry between the two nuclei. Magnetic surfaces are known to act as such a catalyst, but this study finds that electric fields can also induce the spin flips necessary for the interconversion. Molecules made of identical nuclei of non-zero spin exist in nuclear-spin modifications, and the interconversion of these spin isomers is often forbidden for isolated states1,2,3. The interconversion between the nuclear-spin modifications, however, is promoted by inhomogeneous magnetic fields, such as those present on the surfaces of magnetic materials4. Nuclear-spin conversion on diamagnetic and insulating solid substances, on the other hand, is generally considered improbable. Here we present the observation of nuclear-spin flips of H2 and D2 occurring on amorphous solid water surfaces with time constants of 370−140+340 s and 1,220−580+2,980 s, respectively. To explain these unexpected conversion processes, we propose a model of electric-field-induced nuclear-spin flips. In this model, giant and inhomogeneous electric fields present on the ice surface5 mix the electronic states of opposite parities by the Stark effect6, and significantly enhance the spin–orbit couplings between the electronic singlet–triplet spin states of the molecules. By virtue of these effects, the intramolecular hyperfine contact interaction induces the nuclear-spin conversion. This concept should have implications for controlling nuclear magnetization using external electric fields7.

High-resolution storage-ring measurements of the dissociative recombination ofH3+using a supersonic expansion ion source

We have performed measurements of the dissociative electron recombination (DR) of ${\mathrm{H}}_{3}^{+}$ at the ion storage ring TSR utilizing a supersonic expansion ion source. The ion source has been characterized by continuous wave cavity ring-down spectroscopy. We present high-resolution DR rate coefficients for different nuclear spin modifications of ${\mathrm{H}}_{3}^{+}$ combined with precise fragment imaging studies of the internal excitation of the ${\mathrm{H}}_{3}^{+}$ ions inside the storage ring. The measurements resolve changes in the energy dependence between the ortho-${\mathrm{H}}_{3}^{+}$ and para-${\mathrm{H}}_{3}^{+}$ rate coefficients at low center-of-mass collision energies. Analysis of the imaging data indicates that the stored ${\mathrm{H}}_{3}^{+}$ ions may have higher rotational temperatures than previously assumed, most likely due to collisional heating during the extraction of the ions from the ion source. Simulations of the ion extraction shed light on possible origins of the heating process and how to avoid it in future experiments.

Quantum State Reconstruction of a Rotational Wave Packet Created by a Nonresonant Intense Femtosecond Laser Field

We have experimentally determined the amplitudes and phases of a rotational wave packet in an adiabatically cooled benzene molecule, created by a nonresonant intense femtosecond laser field. In this wave-packet reconstruction, the initial wave packet is further interfered by a replica of the first laser pulse, and the resultant modulation in population is observed in a state-resolved manner. Though several states with different nuclear-spin modifications are populated in the initial condition, a single wave packet created from one of them (with J=0) is specifically reconstructed. Phase shifts characteristic of stepwise Raman excitation beyond the perturbative regime are experimentally identified.

Electron collisions and rovibrational action spectroscopy of cold H3+ molecules

Electron recombination of H3+ has found a lot of attention due to its outstanding relevance for the chemistry of the interstellar medium (ISM) and its role as a benchmark for the treatment of dissociative recombination (DR) of polyatomic ions. We report DR measurements performed at the TSR storage ring utilizing a cryogenic ion trap injector. Furthermore, a chemical probing spectroscopy technique is described that allows for a very sensitive monitoring of the populated states inside the ion injector. Since H3+ exists in two different nuclear spin modifications, a controlled manipulation of the ortho/para fraction is needed in order to perform state-selective measurements.

Nuclear Spin Conversion in Molecules

In molecules with identical atoms, such as H, the nuclear spins can take on different arrangements called nuclear spin modifications. For hydrogen, the spin modifications are ortho (with spins aligned parallel) and para (spins aligned antiparallel). In their Perspective, [Hougen and Oka][1] discuss results reported in the same issue by [ Sun et al. ][2] in which conversion among different spin modifications of ethylene is studied. Laser light is used to separate different spin modification to create a gas depleted in one modification, and then the nuclear interconversion is observed over time. Because ethylene has four nuclear spin modifications, the subtle influences of molecular symmetry can be examined in greater detail than previously possible. [1]: http://www.sciencemag.org/cgi/content/full/310/5756/1913 [2]: http://www.sciencemag.org/cgi/content/full/310/5756/1938

The effects of isotope substitution and nuclear spin modifications on the spectra of complexes of tetracene with hydrogen molecules in ultracold 0.37 K He droplets

The van der Waals complexes consisting of single tetracene chromophore molecules with an attached H(2), HD, or a D(2) molecule have been assembled inside cold (0.37 K), large ( approximately 1.5 x 10(4) atoms) helium droplets. Their laser-induced fluorescence spectra exhibit typically three well isolated fairly sharp [deltanu(full width at half maximum) approximately 0.5 cm(-1)] bands in the spectral region 22220-22300 cm(-1). Their positions differ for each isotopomer and also are different for each of the ortho- and para-spin modifications. The common feature (except for D(2)) with the largest redshift at about 30 cm(-1), found also in other related free complexes, is attributed to a strongly bound site above one of the two central benzene rings. The other major features come in pairs spaced 3 cm(-1) apart and are not found in similar gas phase studies. This doublet is assigned to a less tightly bound peripheral site with either slightly different configurations or states of the aduct or possibly the He atoms which are stabilized by the surrounding helium bath. The common feature and one branch of the doublet exhibit a pronounced narrow fine structure with spacings of only 0.1 cm(-1), which is nearly the same for all complexes as well as for the bare chromophore, and maybe be due to partially resolved rotational structure of the bands.

Infrared spectra of carbon monoxide–hydrogen sulfide van der Waals complexes in the C–O stretching region

The CO-stretching vibration–rotation spectra of CO–H 2S, CO–D 2S, and CO–HDS complexes have been studied in the 2150 cm −1 region using a supersonic slit-jet expansion and a tunable diode laser spectrometer. The spectra were analyzed with the help of very recent microwave pure rotational studies of the same complexes. Two bands were assigned for each of the symmetric hydrogen sulfide isotopes, corresponding to the two nuclear spin modifications, para and ortho. The band origins were blue shifted, relative to the free CO molecule, by about 3.8 cm −1 for CO–H 2S and 4.3 cm −1 for CO–D 2S. These are considerably smaller shifts than exhibited by the related CO–water complexes, indicating that the intermolecular forces in CO–H 2S are weaker and more isotropic.

Heat capacity of methane–krypton solid solutions. Conversion effect

The heat capacity of Kr–nCH4 solid solutions with the concentrations n=1; 5; 10% and of the solid solution Kr–1% CH4–0.2% O2 is studied at 0.7–8 K. The contributions Crot to the heat capacity of the solutions due to the rotation of the CH4 molecules are estimated. The deviations of the measured Crot from the values corresponding to the equilibrium distribution of the CH4 nuclear spin modifications are dependent on the correlation between the characteristic times of conversion and of the calorimetric experiment. The effects of temperature, O2 impurities, and CH4 clusters upon the conversion rate are studied. It is shown that the hybrid mechanism of conversion proposed by Berlinsky and Nijman, which takes into account both intramolecular and intermolecular interactions of the proton spins, is predominant.

Electronic spectroscopy and excited state dynamics of the Al-H2/D2 complex.

The electronic spectra of the Al-H2 and Al-D2 complexes are investigated in a collaborative experimental and theoretical study. The complexes were prepared in a pulsed supersonic beam and detected with laser fluorescence excitation spectroscopy. Transitions to bound vibrational levels in electronic states correlating with the excited-state Al(3d, 4p, 4d) + H2/D2 asymptotes were observed by monitoring emission from lower excited Al atomic levels, formed in the non-radiative decay of the excited complex. Fluorescence depletion has also been used to verify that the observed Al-H2 bands all involve the same molecular carrier. The bands have been assigned to the more strongly bound Al-oH2 and Al-pD2 nuclear spin modifications. In contrast to our previous observations for Al(5s)-H2 [X. Yang and P. J. Dagdigian, J. Chem. Phys., 1998, 109, 8920], for which only one potential energy surface (PES) emanates from the dissociation asymptote, the Lorentzian widths of the different vibrational bands in the 3d, 4p, 4d<--3p transitions vary widely, in some cases allowing resolution of the rotational structure of the bands. With the help of the calculated Al(3p)-oH2/pD2 dissociation energies, binding energies of the observed excited vibronic levels are reported. The mechanism of predissociation is investigated theoretically through ab initio calculation of C2 nu cuts of the excited PESs. It is concluded that predissociation occurs through coupling with the repulsive Al(4s)-H2 PES. With these calculations, a qualitative interpretation of the observed bands could be made.