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High-resolution pulsed-field-ionization zero-kinetic-energy photoelectron spectroscopic study of the two lowest electronic states of the ozone cation O3+
Get PDFThe pulsed-field-ionization zero-kinetic-energy (PFI-ZEKE) photoelectron spectrum of jet-cooled O-3 has been recorded in the range 101 000-104 000 cm(-1). The origins of the X (1)A(1)-->X+ (2)A(1) and X (1)A(1)-->A(+) B-2(2) transitions could be determined from the rotational structure of the bands, the photoionization selection rules, the photoionization efficiency curve, and comparison with ab initio calculations. The first adiabatic ionization energy of O-3 was measured to be 101 020.5(5) cm(-1) [12.524 95(6) eV] and the energy difference between the X+ (2)A(1) (0,0,0) and A(+) B-2(2) (0,0,0) states was determined to be DeltaT(0)=1089.7(4) cm(-1). Whereas the X-->X+ band consists of an intense and regular progression in the bending (nu(2)) mode observed up to v(2)(+)=4, only the origin of the X-->A(+) band was observed. The analysis of the rotational structure in each band led to the derivation of the r(0) structure of O-3(+) in the X+ [C-2v,r(0)=1.25(2) A,alpha(0)=131.5(9)degrees] and A(+)[C-2v,r(0)=1.37(5) A,alpha(0)=111.3(38)degrees] states. The appearance of the spectrum, which is regular up to 102 300 cm(-1), changes abruptly at approximate to102 500 cm(-1), a position above which the spectral density increases markedly and the rotational structure of the bands collapses. On the basis of ab initio calculations, this behavior is attributed to the onset of large-amplitude motions spreading through several local minima all the way to large internuclear distances. The ab initio calculations are consistent with earlier results in predicting a seam of conical intersections between the X+ and A(+) states approximate to2600 cm(-1) above the cationic ground state and demonstrate the existence of potential minima at large internuclear distances that are connected to the main minima of the X+ and A(+) states through low-lying barriers
Formation of H2+ and its isotopomers by radiative association: The role of shape and Feshbach resonances
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Previous issue date: 6"The recent observations [1,2] of shape and Feshbach resonances in the high-resolution photoelectron spectra of H, HD and D in the vicinity of the dissociation thresholds of H, HD and D raise questions concerning their potential role in the formation of H and its isotopomers in the early universe by radiative association, a topic of astrophysical interest~[3]._x000d_
Close-coupling calculations for the cross sections of the reactions_x000d_
\begin{align} _x000d_
\text{H}^+ + \text{H} &\to \text{H}_2^+ + h\nu \\_x000d_
\text{H}^+ + \text{D} &\to \text{HD}^+ + h\nu \\_x000d_
\text{D}^+ + \text{H} &\to \text{HD}^+ + h\nu \\_x000d_
\text{D}^+ + \text{D} &\to \text{D}_2^+ + h\nu,_x000d_
\end{align}_x000d_
will be presented which take into account nonadiabatic couplings involving rovibronic and hyperfine interactions, as well as relativistic and radiative corrections._x000d_
The calculated energies and widths will be compared with the experimental results of Ref. [1,2] for H and new data for HD and D. The effect of the resonances on the radiative association rate coefficients will be discussed, also in comparison with earlier studies [4]._x000d_
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\noindent [1] M. Beyer and F. Merkt, \textit{Phys. Rev. Lett.} \textbf{116}, 093001 (2016)._x000d_
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\noindent [2] M. Beyer and F. Merkt, \textit{J. Mol. Spectrosc.} \textbf{330}, 147 (2016)._x000d_
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\noindent [3] Molecule formation in dust-poor environments, J. F. Babb and K. P. Kirby, in ""The molecular astrophysics of stars and galaxies"", T. W. Hartquist and D. A. Williams, eds., Oxford University Press, Oxford, 1998, pp. 11-34._x000d_
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\noindent [4] D. E. Ramaker and J. M. Peek, \textit{Phys. Rev. A} \textbf{13}, 58 (1976).
Observation of heavy Rydberg states in H2 and HD
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Previous issue date: 6The binding energies of the hydrogen atom are given by the Rydberg formula_x000d_
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where the quantum defect vanishes in the case of a pure Coulomb potential. _x000d_
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Heavy Rydberg systems can be realized when the electron is replaced by an anion, which leads in the case of HH to an almost 1000 times larger Rydberg constant and to an infinite number of vibrational states._x000d_
In the diabatic molecular basis, these ion-pair states are described by long-range Coulomb potentials with and symmetry._x000d_
In this basis, the level energies are described by an almost energy-independent, nonzero quantum defect, reflecting the finite size of H. Strong interactions at small internuclear distances lead to strong variation of with ._x000d_
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Gerade [2] and ungerade [3] ion-pair states have been observed in H with principal quantum numbers up to . The quantum defects in this range were found to vary with energy, indicating the inadequacy of a pure diabatic picture._x000d_
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Spectra of ungerade heavy Rydberg states of H with showing that the quantum defect only becomes energy independent for will be presented, supporting the description using a diabatic basis._x000d_
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I will also present first observations of ion-pair states in HD, showing two series of heavy Rydberg states, HD and HD, which have different series limits._x000d_
The experimental results will be discussed and compared with calculations using both an adiabatic and a diabatic basis. _x000d_
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[1] S. Pan, and F. H. Mies, textit{J. Chem. Phys.} textbf{89}, 3096 (1988)._x000d_
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[2] M. O. Vieitez, T. I. Ivanov, E. Reinhold, C. A. de Lange, and W. Ubachs, textit{Phys. Rev. Lett.} textbf{101}, 163001 (2008)._x000d_
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[3] R. C. Ekey, and E. F. McCormack, textit{Phys. Rev. A} textbf{84}, 020501(R) (2011)._x000d
High resolution photofragment translational spectroscopy using Rydberg tagging methods
No full textPhotofragment translational spectroscopy (PTS) is a general and versatile technique for exploring the dynamics of gas-phase molecular photodissociation processes, through measurement of the speeds and directions of the resulting atomic and molecular products. This article describes recent enhancements in the achievable kinetic energy resolution and then focuses on applications employing one particular high-resolution variant of the technique, wherein H(D) atom photofragments are monitored by Rydberg tagging methods. Systems described range from small hydrides such as H2O, NH3, and C2H2 to larger heteroaromatic molecules such as pyrrole and phenol. In the former cases, the achievable resolution allows observation of individual rovibrational quantum states of the radical partner, providing detailed insights into the forces acting during the dissociation process. This allows intimate contact with complementary ab initio and wave packet modeling studies, and accurate determination of bond strengths and related thermodynamic quantities. The density of possible product states increases rapidly for larger radical cofragments. PTS studies of several heteroaromatics point to the generality of prompt dissociation from 1πσ* excited states, which manifests itself in remarkably mode specific fragmentation dynamics. The article concludes with a summary of complementary photofragmentation studies of hydride radicals and an anticipation of future activities in this field
High-resolution spectroscopy of He2+ using rydberg-series extrapolation and zeeman-decelerated supersonic beams of metastable He2
Get PDFHaving only three electrons, He represents a system for which highly accurate \emph{ab initio} calculations are possible. The latest calculations of rovibrational energies in He do not include relativistic or QED corrections but claim an accuracy of 120\,MHz\footnote{W.-C. Tung, M. Pavanello and L. Adamowicz, J. Chem. Phys. \textbf{136}, 104309 (2012).}. We have performed high-resolution Rydberg spectroscopy of metastable He molecules\footnote{P. Jansen, L. Semeria, L. Esteban Hofer, S. Scheidegger, J.A. Agner, H. Schmutz, and F. Merkt, Phys. Rev. Lett. \textbf{115}, 133202 (2015).} and employed multichannel-quantum-defect-theory extrapolation techniques\footnote{D. Sprecher, J. Liu, T. Krähenmann, M. Schäfer, and F. Merkt, J. Chem. Phys. \textbf{140}, 064304 (2014).} to determine the rotational energy-level structure in the He ion. To this end, we have produced samples of metastable helium molecules in supersonic beams with velocities tunable down to 100\,m/s by combining a cryogenic supersonic-beam source with a multistage Zeeman decelerator\footnote{M. Motsch, P. Jansen, J. A. Agner, H. Schmutz, and F. Merkt, Phys. Rev. A \textbf{89}, 043420 (2014).}. The metastable He molecules are excited to Rydberg states using the frequency-doubled output of a pulse-amplified ring dye laser. Although the bandwidth of the laser system is too large to observe the reduction of the Doppler width resulting from deceleration, the deceleration greatly simplifies the spectral assignments because of its spin-rotational state selectivity. Our approach enabled us to determine the rotational structure of He with an unprecedented accuracy of 18\,MHz, to quantify the size of the relativistic and QED corrections by comparison with the results of Tung \emph{et al.} and to precisely measure the rotational structure of the metastable state for comparison with the results of Focsa \emph{et al.}\footnote{C. Focsa, P. F. Bernath, and R. Colin, J. Mol. Spectrosc. \textbf{191}, 209 (1998).}. Here, we present an extension of these measurements in which we have measured higher rotational intervals of He. In addition, we have replaced the pulsed UV laser by a cw UV laser and improved the resolution of the spectra by a factor of more than five\footnote{P. Jansen, L. Semeria, and F. Merkt, J. Mol. Spectrosc. \textbf{322}, 9 (2016).}.Made available in DSpace on 2017-01-26T21:37:07Z (GMT). No. of bitstreams: 3
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Previous issue date: 2016-06-2
High‐resolution Photoelectron Spectroscopy
No full textThe evolution of photoelectron spectroscopy of neutral molecules in the gas phase, from its first applications in the early 1960s to the present day, is reviewed with emphasis on the description of methods that have contributed to the improvement of resolution. In the early days, photoelectron spectroscopy was very successfully used to study the electronic structure of molecules. As the resolution improved, the emphasis gradually shifted to studies of the structure and dynamics of molecular cations. Current methods enable one to fully resolve the rotational structure in the photoelectron spectra of polyatomic molecules and even to measure the fine and hyperfine structures of positively charged atoms and small molecules. Several of the methods providing high resolution make use of the long lifetimes and electric‐field‐ionization properties of high Rydberg states and give access to detailed information on molecular ionization channels and molecular photoionization
Experimental Methods in Cation Spectroscopy
No full textThe spectroscopic study of molecular cations in the gas phase poses a range of challenges which are generally associated with their high chemical reactivity, the difficulties in producing sufficiently high number densities of the desired ion species in a spectrometer, and the limits on the particle densities imposed by the space‐charge effect. The present article provides an overview of different experimental techniques that have been developed over the past years to overcome these problems. The topics discussed include highly sensitive laser‐based spectroscopic methods (cavity ring‐down spectroscopy, frequency‐, velocity‐ and concentration‐modulation schemes, and various types of action spectroscopy), the production of internally cold samples of ions and the use of mass‐spectrometric methods to select the species of interest out of a mixed sample. Owing to these developments, the range of molecular cations amenable to a spectroscopic characterization has been extended considerably over the past years and now includes various types of cluster ions, protonated biomolecules, ionic carbon chains, and the ions of polycyclic aromatic hydrocarbons
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Determination of the spin-rotation fine structure of He2+
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Previous issue date: 6Measuring spin-rotation intervals in molecular cations is challenging, particularly so when the ions do not have electric-dipole-allowed rovibrational transitions. We present a method to determine the spin-rotational fine structure of molecular ions from the fine structure of high Rydberg statesa . The method is illustrated by the determination of the so far unknown spin-rotation fine structure of the fundamentally important He2 + ion in the X+ 2Σ + u ground electronic state. The interaction that is responsible for the level structure in the high Rydberg states of He2 that were probed in our experiment is the n-independent spin-rotation interaction of the ion core. As a consequence, the fine-structure splittings in He2 + can be related to the fine-structure of the Rydberg states by applying an angular-momentum basis transformation from Hund’s case (e[b]) to Hund’s case (d).
The experiment relies on the use of single-mode cw radiation to record spectra of high Rydberg states of He2 from the a 3Σ + u metastable state. Metastable helium molecules are produced by striking a discharge in a pulsed expansion of neat helium gasb . Cooling the valve body to a temperature of 10 K and using continuous-wave excitation results in an observed Doppler-limited linewidth of 25 MHz. The fine structure of Rydberg states of He2 is determined from strict selection rules by comparing the observed splitting of the Rydberg spectrum with the spin-rotational intervals of the initial metastable state. The fine-structure splittings of the v + = 0, N + = 1, 3, and 5 levels of He2 + are 7.96(14)MHz, 17.91(32) MHz and 28.0(6) MHz, respectively
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