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A systematic study involving the use and optimization of gas-phase modifiers in quantitative differential mobility-mass spectrometry (DMS-MS) analysis is presented using nucleoside-adduct biomarkers of DNA damage as an important reference point for analysis in complex matrices. Commonly used polar protic and polar aprotic modifiers have been screened for use against two deoxyguanosine adducts of DNA: N-(deoxyguanosin-8-yl)-4-aminobiphenyl (dG-C8-4-ABP) and N-(deoxyguanosin-8-y1)-2-amino-l-methyl-6-phenylimidazo[4,5-b]pyridine (dG-C8-PhIP). Particular attention was paid to compensation voltage (CoV) shifts, peak shapes, and product ion signal intensities while optimizing the DMS-MS conditions. The optimized parameters were then applied to rapid quantitation of the DNA adducts in calf thymus DNA. After a protein precipitation step, adduct levels corresponding to less than one modification in 10(6) normal DNA bases were detected using the DMS-MS platform. Based on DMS fundamentals and ab initio thermochemical results, we interpret the complexity of DMS modifier responses in terms of thermal activation and the development of solvent shells. At very high bulk gas temperature, modifier dipole moment may be the most important factor in cluster formation and cluster geometry, but at lower temperatures, multi-neutral clusters are important and less predictable. This work provides a useful protocol for targeted DNA adduct quantitation and a basis for future work on DMS modifier effects.

We report calculations of vibrational autoionization rates of CaF Rydberg states, based on the results of a global multi-channel quantum defect theory (MQDT) fit. Our goal is to use intuitive physical models to interpret and extend the results from the MQDT calculations and, in particular, to characterize the physical mechanisms for the interaction between the Rydberg electron and the ion-core. The calculations indicate that, among the six strongly l-mixed core-penetrating (CP) Rydberg series of CaF, the n.36 p ^ Π Rydberg series has the fastest Δ v = 1 vibrational autoionization rate, which is at least four times larger than that for the other CP Rydberg series, in agreement with experimental results. We first demonstrate that the rotational level dependence of the vibrational autoionization rate of the n.36 p ^ Π series is satisfactorily explained by l-uncoupling interactions, which differ for the positive and negative Kronig symmetry levels. Next, we interpret the relative vibrational autoionization rates of all six CP Rydberg series in the context of a valence-precursor (VP) model. The VP model is a consequence of Mulliken's rule, which states that the innermost lobe of the Rydberg wavefunction remains invariant in both the nodal position and shape for members of the same Rydberg series. The electronic properties of the six VP states, which are the terminus states (lowest-n) of each of the six CP Rydberg series, are further characterized in terms of a ligand-field model, providing insight into the intimate relationship between the Rydberg electron density in the ion-core region and the vibrational autoionization rate. [ABSTRACT FROM AUTHOR]

In differential mobility spectrometry (also referred to as high-field asymmetric waveform ion mobility spectrometry), ions are separated on the basis of the difference in their mobility under high and low electric fields. The addition of polar modifiers to the gas transporting the ions through a differential mobility spectrometer enhances the formation of clusters in a field-dependent way and thus amplifies the high- and low-field mobility difference, resulting in increased peak capacity and separation power. Observations of the increase in mobility field dependence are consistent with a cluster formation model, also referred to as the dynamic cluster decluster model. The uniqueness of chemical interactions that occur between an ion and cluster-forming neutrals increases the selectivity of the separation, and the depression of low-field mobility relative to high-field mobility increases the compensation voltage and peak capacity. The effect of a polar modifier on the peak capacity across a broad range of chemicals has been investigated. We discuss the theoretical underpinnings which explain the observed effects. In contrast to the result with a polar modifier, we find that using mixtures of inert gases as the transport gas improves the resolution by reducing the peak width but has very little effect on the peak capacity or selectivity. The inert gas helium does not cluster and thus does not reduce low-field mobility relative to high-field mobility. The observed changes in the differential mobility u parameter exhibited by different classes of compounds when the transport gas contains a polar modifier or has a significant fraction of inert gas can be explained on the basis of the physical mechanisms involved in the separation processes. 10.1021/ac902571u

A microfabricated planar differential ion mobility spectrometer operating from 0.4 to 1.55 atm in a supporting atmosphere of purified air was used to characterize the effects of pressure and electric field strength on compensation voltage, ion transmission, peak width, and peak intensity in differential mobility spectra. Peak positions, in compensation voltage as a function of separating rf voltage, were found to vary with pressure in a way that can be simplified by expressing both compensation and separation fields in Townsend units for E/N. The separation voltage providing the greatest compensation voltage and the greatest resolution is ion-specific but often occurs at E/N values that are unreachable at elevated pressure because of electrical breakdown. The pressure dependence of air breakdown voltage near 1 atm is sublinear, allowing higher E/N values to be reached at reduced pressure, usually resulting in greater instrumental resolution. Lower voltage requirements at reduced pressure also reduce device power consumption.

The dependence of multipole moments and polarizabilities on external fields appears in many applications including biomolecular molecular mechanics, optical non-linearity, nanomaterial calculations, and the perturbation of spectroscopic signatures in atomic clocks. Over a wide range of distances, distributed multipole and polarizability potentials can be applied to obtain the variation of atom-centered atoms-in-molecules electric properties like bonding-quenched polarizability. For cylindrically symmetric charge distributions, we examine single-center and atom-centered effective polarization potentials in a non-relativistic approximation for Rydberg states. For ions, the multipole expansion is strongly origin-dependent, but we note that origin-independent invariants can be defined. The several families of invariants correspond to optimized representations differing by origin and number of terms. Among them, a representation at the center of dipole polarizability optimizes the accuracy of the potential with terms through 1/r4. We formulate the single-center expansion in terms of polarization-modified effective multipole moments, defining a form related to the source-multipole expansion of Brink and Satchler. Atom-centered potentials are an origin independent alternative but are limited both by the properties allowed at each center and by the neglected effects like bond polarizability and charge flow. To enable comparisons between single-center effective potentials in Cartesian or spherical form and two-center effective potentials with differing levels of mutual induction between atomic centers, we give analytical expressions for the bond-length and origin-dependence of multipole and polarizability terms projected in the multipole and polarizability expansion of Buckingham. The atom-centered potentials can then be used with experimental data and ab initio calculations to estimate atoms-in-molecules properties. Some results are given for BaF+ and HF showing the utility and limitations of the approach. More detailed results on X ¹∑+ CaF+ are published separately. [ABSTRACT FROM AUTHOR]

A technique known as Hybrid Linear Pattern Analysis is presented, This technique is a combination of model-based and pattern recognition-based approaches to the analysis of spectroscopic data.

Because of their central importance in chemistry and biology, water molecules have been the subject of decades of intense spectroscopic investigations. Rotational spectroscopy of water vapor has yielded detailed information about the structure and dynamics of isolated water molecules, as well as water dimers and clusters. Nonlinear rotational spectroscopy in the terahertz regime has been developed recently to investigate the rotational dynamics of linear and symmetric-top molecules whose rotational energy levels are regularly spaced. However, it has not been applied to water or other lower-symmetry molecules with irregularly spaced levels. We report the use of recently developed two-dimensional (2D) terahertz rotational spectroscopy to observe high-order rotational coherences and correlations between rotational transitions that were previously unobservable. The results include twoquantum (2Q) peaks at frequencies that are shifted slightly from the sums of distinct rotational transitions on two different molecules. These results directly reveal the presence of previously unseen metastable water complexes with lifetimes of 100 ps or longer. Several such peaks observed at distinct 2Q frequencies indicate that the complexes have multiple preferred bimolecular geometries. Our results demonstrate the sensitivity of rotational correlations measured in 2D terahertz spectroscopy to molecular interactions and complexation in the gas phase. [ABSTRACT FROM AUTHOR]

We present an improved quantum defect theory model for the 's,' 'p,' 'd,' and 'f' Rydberg series of CaF. The model, which is the result of an exhaustive fit of high-resolution spectroscopic data, parameterizes the electronic structure of the ten ('s'Σ, 'p'Σ, 'p'Π, 'd'Σ, 'd'Π, 'd'Δ, 'f'Σ, 'f'Π, 'f'Δ, and 'f'[uppercase_phi_synonym]) Rydberg series of CaF in terms of a set of twenty μℓℓ′(Λ) quantum defect matrix elements and their dependence on both internuclear separation and on the binding energy of the outer electron. Over 1000 rovibronic Rydberg levels belonging to 131 observed electronic states of CaF with n* ≥ 5 are included in the fit. The correctness and physical validity of the fit model are assured both by our intuition-guided combinatorial fit strategy and by comparison with R-matrix calculations based on a one-electron effective potential. The power of this quantum defect model lies in its ability to account for the rovibronic energy level structure and nearly all dynamical processes, including structure and dynamics outside of the range of the current observations. Its completeness places CaF at a level of spectroscopic characterization similar to NO and H2. [ABSTRACT FROM AUTHOR]

The Stark effect in molecular Rydberg states is qualitatively different from the Stark effect in atomic Rydberg states because of the anisotropy of the ion core and the existence of rotational and vibrational degrees of freedom. These uniquely molecular features cause the electric-field-induced decoupling of the Rydberg electron from the body frame to proceed in several stages in a molecule. Because the transition dipole moment among the same-n* Rydberg states is much larger than the permanent dipole moment of the ion core, the decoupling of the Rydberg electron from the ion core proceeds gradually. In the first stage, analyzed in detail in this paper, l and N are mixed by the external electric field, while N+ is conserved. In the further stages, as the external electric field increases, N+, n*, and v+ are expected to undergo mixing. We have characterized these stages in n*=13, v+=1 states of CaF. The large permanent dipole moment of CaF+ makes CaF qualitatively different from the other molecules in which the Stark effect in Rydberg states has been described (H2, Na2, Li2, NO, and H3) and makes it an ideal testbed for documenting the competition between the external and CaF+ dipole electric fields. We use the weak-field Stark effect to gain access to the lowest-N rotational levels of f, g, and h states and to assign their actual or nominal N+ quantum number. Lowest-N rotational levels provide information needed to disentangle the short-range and long-range interactions between the Rydberg electron and the ion core. We diagonalize an effective Hamiltonian matrix to determine the l-characters of the 3≤=l≤=5 core-nonpenetrating 2Σ+ states and to characterize their mixing with the core-penetrating states. We conclude that the mixing of the l=4, N-N+=-4(g(-4)) state with... [ABSTRACT FROM AUTHOR]

Observation and analysis of the f(ℓ=3), g(ℓ=4), and h(ℓ=5) Rydberg series of CaF in the range 13≤=n≤=17 is presented. Simultaneous analysis of the f, g, and h Rydberg series of CaF, combined with significant improvements to the long-range model for nonpenetrating Rydberg states, provides a generally applicable scheme for separating the effects of the long-range (electric multipole and polarization) and short-range (core penetration) interactions between the Rydberg electron and ion core in diatomic molecules. Techniques for rigorous assignment of nonpenetrating states, as well as extensions of the long-range model, are discussed. Explicit formulas for the first- and second-order matrix elements of the first four anisotropic electric multipole interactions (electric dipole, quadrupole, octupole, and hexadecapole) are given. The discrepancies between the observed behavior and that predicted by the long-range model, which are particularly significant for the f series, are shown to be due to penetration of the Rydberg electron wavefunction within the ion core. We show that these penetration effects can be understood within the framework of ligand field theory and conclude with a discussion of the relative contributions of the long- and short-range interactions to the quantum defects of the core-penetrating “s,” “p,” and “d” series of CaF. [ABSTRACT FROM AUTHOR]

Results of ab initio R-matrix calculations [S. N. Altunata et al., J. Chem. Phys. 123, 084319 (2005)] indicate the presence of a broad shape resonance in electron-CaF+ scattering for the 2Σ+ electronic symmetry near the ionization threshold. The properties of this shape resonance are analyzed using the adiabatic partial-wave expansion of the scattered electron wave function introduced by Le Dourneuf et al. [J. Phys. B 15, L685 (1982)]. The qualitative aspects of the shape resonance are explained by an adiabatic approximation on the electronic motion. Mulliken’s rule for the structure of the Rydberg state wave functions [R. S. Mulliken, J. Am. Chem. Soc. 86, 3183 (1964)] specifies that, except for an (n*)-3/2 amplitude scale factor, every excited state wave function within one Rydberg series is built on an innermost lobe that remains invariant in shape and nodal position as a function of the excitation energy. Mulliken’s rule implies a weak energy dependence of the quantum defects for an unperturbed molecular Rydberg series, which is given by the Rydberg-Ritz formula. This zero-order picture is violated by a single 2Σ+ CaF Rydberg series at all Rydberg state energies (n*=5→∞, more so with increasing n*) below the ionization threshold, under the broad width of the shape resonance. Such a violation is diagnostic of a global “scarring” of the Rydberg spectrum, which is distinct from the more familiar local level perturbations. [ABSTRACT FROM AUTHOR]

We report a new form of microwave optical double-resonance spectroscopy called millimeter-wave-detected, millimeter-wave optical polarization spectroscopy (mmOPS). In contrast to other forms of polarization spectroscopy, in which the polarization rotation of optical beams is detected, the mmOPS technique is based on the polarization rotation of millimeter waves induced by the anisotropy from optical pumping out of the lower or upper levels of the millimeter wave transition. By monitoring ground-state rotational transitions with the millimeter waves, the mmOPS technique is capable of identifying weak or otherwise difficult-to-observe optical transitions in complex chemical environments, where multiple molecular species or vibrational states can lead to spectral congestion. Once a transition is identified, mmOPS can then be used to record pure rotational transitions in vibrationally and electronically excited states, with the resolution limited only by the radiative decay rate. Here, the sensitivity of this nearly-background-free technique is demonstrated by optically pumping the weak, nominally spin-forbidden CS e 3Σ--X 1Σ+ (2-0) and d 3Δ-X 1Σ+ (6-0) electronic transitions while probing the CS X 1Σ+ (v″=0,J″=2-1) rotational transition with millimeter waves. The J′=2,N′=2←J′=1,N′=1 pure rotational transition of the CS e 3Σ- (v′=2) state is then recorded by optically preparing the J′=1,N′=1 level of the e 3Σ- (v′=2) state via the J′=1,N′=1←J″=1 transition of the e 3Σ--X 1Σ+ (2-0) band. [ABSTRACT FROM AUTHOR]

The ab initio K matrix method described in the preceding paper (Part I) is applied to the Rydberg electronic structure of calcium monofluoride. The spectroscopic quantum defects for the 2Σ+, 2Π, 2Δ, and 2[uppercase_phi_synonym] states of CaF are computed using the effective potential of Arif et al. [M. Arif, Ch. Jungen, and A. L. Roche, J. Chem. Phys. 106, 4102 (1997)]. Satisfactory agreement with the experimental values is obtained. The eigenquantum defects obtained from the reaction matrix for the CaF++e- system are found to be strongly energy dependent. The analysis shows that the main features of the energy-dependent structure in the eigenphases are a consequence of a broad molecular shape resonance. Partial-l (orbital angular momentum) characters of two interacting collision eigenchannels vary rapidly as a function of increasing collision energy. This prominent variation leads to interference structure in the intensities for transitions into the ionization continuum, manifesting nodal points in the total ionization cross section in the continuum above the shape resonance. The usefulness of this structure in the ionization cross section as a direct probe of the l-character of the bound state is discussed. In addition, ab initio results for the photoelectron angular distribution and the anisotropy parameter are presented. These computed results are susceptible to direct experimental verification. [ABSTRACT FROM AUTHOR]

An ab initio R-matrix method for determining the molecular reaction matrix of scattering theory is introduced. The method makes use of a principal-value Green function to compute the collision channel wave functions for the scattered electron, in combination with the Kohn variational scheme for the evaluation of R-matrix eigenvalues on a spherical boundary surface at short range. This technique permits the size of the bounded volume in the variational calculation to be reduced, making the computations fast and efficient. The reaction matrix is determined in a form that minimizes its energy dependence. Thus the procedure does not require modification or an increase in the computational effort to study the electronic structure and dynamics in Rydberg molecules with extremely polar ion cores. The analysis is specialized to examine the bound-state and free-electron scattering properties of nearly one-electron molecular systems, which are characterized by a Rydberg/scattering electron incident on a closed-shell ion core. However, it is shown that the treatment is compatible with all-electron/ab initio representations of open-shell and nonlinear polyatomic ion cores, emphasizing its generality. The introduced approach is used to calculate the electronic spectrum of the calcium monofluoride molecule, which has the extremely polar (Ca+2F-)+e- closed-shell ion-core configuration. The calculation utilizes an effective single-electron potential determined by M. Arif, C. Jungen, and A. L. Roche [J. Chem. Phys. 106, 4102 (1997)] previously. Close agreement with experimental data is obtained. The results demonstrate the practical utility of this method as a viable alternative to the standard variational approaches. [ABSTRACT FROM AUTHOR]

High-resolution (0.013 cm-1) spectra of the second overtone of the OH stretch of trans-nitrous acid were recorded between 10 230 and 10 350 cm-1. The spectra were analyzed to yield a complete set of rotational parameters, including the A, B, and C rotational constants and the 5 quartic distortion constants. Two groups of anharmonic perturbations were observed and analyzed, and for one of these groups, the spectra were fit with a second model to extract an improved set of parameters that took into account these interactions. Much less information could be obtained from our spectra about the second group of perturbations, except that the same dark vibrational state was probably not responsible for both groups of perturbations. The anharmonic coupling matrix element between the 3ν[sub 1] “bright” state and one “dark” vibrational state was found to be 0.0338(31) cm-1, and possible assignments for the “dark” level are discussed. In addition, we report the first measurement of the absolute intensity of the 3ν[sub 1] overtone band, which was found to be 2.1±0.6×10[sup -21] cm[sup 2] molecule[sup -1] cm[sup -1]. Our findings are discussed in the context of the contribution of overtone-mediated processes to OH radical production in the upper atmosphere. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We propose a stability criterion applicable to eigenstates of approximate Hamiltonians. We define a ‘‘robustness’’ of the physical properties of the underlying system under a variety of internal and external conditions that can be deduced from stability properties of the eigenstates of the approximate Hamiltonian. Stability properties are assigned to an ensemble of eigenstates according to the statistics of its response to an ensemble of random perturbations of given magnitude. The stability criterion is explicitly formulated for the eigenstates of polyatomic molecules exhibiting a normal to local mode transition. As an example, a stability analysis for the water molecule is carried out and experiments in the low density gas phase (rotational energy transfer), high density gas phase (collision-induced spectra), and condensed phase are suggested. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A time-resolved infrared double-resonance technique has been used to measure vibrationally and rotationally inelastic collision rates in ground and vibrational overtone levels of methane. A Raman-shifted Ti:sapphire laser is used to pump J=0 through 7 states in the 2ν3 and ν3+ν4 levels of 12CH4, and a tunable diode laser is used to probe the time-dependent level populations. Vibrational equilibration is observed among the octad, pentad, and dyad levels, with subsequent relaxation to the ground state. State-to-state rotational energy transfer rates are obtained in the ground and ν3+ν4 excited vibrational levels, and compared with theoretical predictions and with pressure-broadening measurements on the corresponding transitions. The probability of molecular reorientation in an inelastic collision is also inferred from the polarization dependence of the relaxation times. Parity-conserving and vibrational angular momentum propensity rules are inferred for the lower rotational levels of methane. © 1994 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We argue that the physical significance of the normal to local mode transition in triatomic molecules AB2 lies in an enhanced susceptibility of eigenstates to symmetry breaking perturbations. States of local character have higher susceptibilities than states of normal character. The quantum mechanical condition for the normal to local mode transition is formulated. The issue of stability is addressed from a classical, semiclassical, and quantum mechanical point of view. The question of instability of the unperturbed quantum system is decided individually for each eigenstate. The relevance for experiments is outlined. [ABSTRACT FROM AUTHOR]

This paper reports analytical approximations to the level-spacing distributions for two types of eigenspectra: a Gaussian orthogonal ensemble (GOE) eigenspectrum, a fraction of whose levels are omitted, and the superposition of a fractional GOE distribution with levels from a random sequence. The combined distribution would be expected when a spectrum with a GOE spacing distribution is observed, but with insufficient dynamic range to observe all the lines, and with the possibility of contamination from other sequences or from noise identified as spectral lines. Berry recently demonstrated that GOE type spacing statistics are expected in the semiclassical limit of a classically chaotic Hamiltonian. Therefore, the present formula may prove useful in testing for quantum signatures of classical chaos in quantum molecular spectra. [ABSTRACT FROM AUTHOR]

Optical transitions to the 2B2 electronic state region of NO2 have been observed in double resonance with three millimeter-wave ground state rotational transitions. The probed ground state transitions were the N=2 (F1), N=4 (F1), and N=10 (F2), K=0→1 Q branch transitions near 250 GHz. The optical range scanned was 16 800 to 16 950 cm-1. The 100,10–101,9 F2 spectrum was combined with previous double resonance results with the ground state 91,9–100,10 F1 transition. This comparison shows 70 out of 160 upper states with J=19/2 to have indefinite or mixed N values. It establishes violations of the ΔN=ΔJ selection rule in this spectral region which contains bands with a wide range of spin-rotation constants. Lack of intensity correlation between the two spectra suggests two classes of upper states are present: a group of strongly N-mixed upper states, and a group essentially unmixed in N. Heller’s F parameter, which measures the fraction of all possible symmetry-restricted basis states contributing to a typical eigenstate, is similar for the double-resonance spectra of the three N values, but is greater for K=1 than for K=0. By finding upper states in common, R lines from N=2 and P lines from N=4 have been identified. [ABSTRACT FROM AUTHOR]