Rotation-Induced Breakdown of Torsional Quantum Control

Control of the torsional angles of nonrigid molecules is key for the development of emerging areas like molecular electronics and nanotechnology. Based on a rigorous calculation of the rotation-torsion-Stark energy levels of nonrigid biphenyl-like molecules, we show that, unlike previously believed, instantaneous rotation-torsion-Stark eigenstates of such molecules, interacting with a strong laser field, present a large degree of delocalization in the torsional coordinate even for the lowest energy states. This is due to a strong coupling between overall rotation and torsion leading to a breakdown of the torsional alignment. Thus, adiabatic control of changes on the planarity of this kind of molecule is essentially impossible unless the temperature is on the order of a few Kelvin

LARGE AMPLITUDE MOTION EFFECTS IN THE TPES SPECTRUM OF METHYL ISOCYANATE

Methyl isocyanate is a non-rigid quasi-symmetric top molecule displaying a torsion of its methyl group and a large amplitude \ce{CNC} bending mode.\footnote{\label{koput}Kr\c{e}glewski, {\em J.\ Molec.\ Spectrosc.}~{\bf 105} (1984) 8; and Koput, {\em ibid.}~{\bf 106} (1984) 12} The torsion is nearly free as the hindering barrier is only 20~cm$^{-1}$. The barrier to linearity, also very low, is on the order of 920~cm$^{-1}$. Although the $a$-type transitions of methyl isocyanate have already been recorded up to the submillimeter wave domain,\footnote{Koput, {\em J.\ Molec.\ Spectrosc.}~{\bf 115} (1986) 131; and Cernicharo {\em et al.,} {\em A\&A}~{\bf 587} (2016) L4} spectroscopic information is still lacking, especially concerning the cation. Threshold photoelectron spectroscopy has been used to obtain spectroscopic information on the cationic species (\ce{CH3NCO+}) of methyl isocyanate. The spectrum recorded from 84000 to 94000~cm$^{-1}$ (10.4 to 11.6~eV) using VUV synchrotron radiation displays several sharp features superimposed on a broad feature spanning nearly 8000~cm$^{-1}$. As shown by the {\em ab initio} calculations carried out in this work, the ground electronic state of the cation is doubly degenerate and is split into a lower $\widetilde{X}^+$ and an upper $\widetilde{A}^+$ substate by vibronic couplings. The ground electronic state of the neutral and the $\widetilde{X}^+$ substate of the cation are characterized by similar values of the methyl group internal rotation barrier. As for the $\widetilde{A}^+$ substate of the cation, a much larger value shifted by $\pi/3$ was calculated. Accounting for the two large amplitude motions and for the overall rotation,\textsuperscript{$a$} a calculation of the rovibronic energies of the neutral and the cationic species is in progress and should allow us to model the TPES spectrum. This calculation relies on Gaussian quadrature to treat the singularity at the linear configuration.\footnote{Coudert, Gans, Holzmeier, Loison, Garcia, Alcaraz, Lopes, and R\"{o}der, {\em J.\ Chem.\ Phys.}~{\bf 149} (2018) 224304} The strong dependence on the methyl group internal rotation barrier on the \ce{CNC} bending angle is also taken into account.\textsuperscript{$a$} In the talk, the results of the rovibronic energies calculation will be reported and the experimental TPES spectrum will be compared to the theoretical one

GLOBAL ANALYSIS OF THE RADICAL ROTATIONAL, VIBRATIONAL AND ELECTRONIC TRANSITIONS

The \ce{NH2} radical, first observed by Herzberg and Ramsay,\footnote{Herzberg and Ramsay, {\em J.\ Chem.\ Phys.}~{\bf 20} (1952) 347} is dominated by a strong Renner-Teller effect\footnote{Dressler and Ramsay, {\em Phil.\ Trans.\ R.\ Soc.\ A}~{\bf 25} (1959) 553} giving rise to two electronic states: a bent \tilde{X}\, ^2B_1 ground state and a quasi-linear \tilde{A}\, ^2A_1 excited state. The \ce{NH2} radical has been the subject of numerous high-resolution investigations and its rotational, vibrational, and electronic transitions\footnote{\label{refs}Hadj Bachir, Huet, Destombes, and Vervloet, {\em J.\ Molec.\ Spectrosc.}~{\bf 193} (1999) 326; McKellar, Vervloet, Burkholder, and Howard, {\em ibid.}~{\bf142} (1990) 319; Morino and Kawaguchi, {\em ibid.}~{\bf 182} (1997) 428; and Martin-Drumel, Pirali, and Vervloet, {\em J.\ Phys.\ Chem.\ A}~{\bf 118} (2014) 1331} have been measured. In the most recent investigation,\textsuperscript{$c$} a value of the rotational quantum number $N$ as large as 26 could be reached and the line position analysis revealed that, even in the \tilde{X}\, ^2B_1 ground electronic state, an anomalous centrifugal distortion occurs and originates from the strong coupling between the overall rotation and the bending $\nu_2$ mode, as in the case of the water molecule.\footnote{Camy-Peyret and Flaud, {\em Molec.\ Phys.}~{\bf 32} (1976) 523} Two theoretical approaches accounting for the Renner-Teller effect are setup to compute the rovibronic energy levels of \ce{NH2}. The first one is an effective approach in which the large amplitude bending mode and the overall rotation are treated simultaneously.\footnote{Coudert, Gans, Garcia, and Loison, {\em J. Chem. Phys.}~{\bf 148} (2018) 054302} The second one is an exact approach, based on a tridimensional potential energy surface, in which all three vibrational modes are considered in addition to the overall rotation.\footnote{\label{moi}Coudert, Gans, Holzmeier, Loison, Garcia, Alcaraz, Lopes, and R\"oder, {\em J.\ Chem.\ Phys.}~{\bf 149} (2018) 224304} In the talk, both approaches will be tested fitting experimental high-resolution data pertaining to the \ce{NH2} radical. The first approach will be applied to data involving the ground and (010) vibrational states. The second approach should allow us to treat any vibrational states and to adjust the tridimensional potential energy surface of the radical.\footnote{Jensen, Odaka, Kraemer, Hirano and Bunker, {\em Spectrochim.\ Acta Part A}~{\bf 58} (2002) 763

MICROWAVE SPECTRUM OF 1-ADAMANTANOL C10H15-OH

1-Adamantanol is a heavy non-rigid molecule consisting of 1-adamantyl and hydroxyl groups. Internal rotation about the 1-adamantyl 3-fold axis of symmetry was evidenced some time ago\footnote{Craven, {\em Spectrochim.\ Acta,}~{\em 29A} (1973) 679} leading to an estimated value of the $A$-$E$ splitting of 10~cm$^{-1}$. The microwave spectrum of 1-adamantanol was recorded later\footnote{Corbelli, Degli Esposti, Favero, and Lister, {\em J.\ Chem.\ Soc.\ Trans.~2,}~{\bf 83} (1987) 2225} in the 8 to 40 GHz region. Even though individual rotational lines could not be assigned, a value of 410~cm$^{-1}$ was obtained for $V_3$ the height of the barrier hindering the internal rotation. A cold molecular beam and a room temperature submillimeter wave spectra of 1-adamantanol were recorded in the 2--12 and 140--220~GHz ranges, respectively. 1404 parallel $a$-type transitions have been assigned in both spectra. A line frequency analysis of this new data set and of the perpendicular $b$-type clusters previously observed$^b$ was carried out using an IAM approach.\footnote{Hougen, {\em J.\ Mol.\ Spectrosc.,}~{\bf 114} (1985) 395; and Coudert and Hougen, {\em ibid,}~{\bf 130} (1988) 86} In the paper, the new data and the results of the analysis will be presented. As 1-adamantanol is a nearly symmetric top molecule with an asymmetry parameter$^b$ $\kappa$ close to $-0.99$, asymmetry splittings could not be resolved in the new spectra and $B-C$ was set to zero. Owing to the fact that the moment of inertia of 1-adamantyl about the axis of internal rotation is 400 times larger than that of the OH group about the same axis, $\rho$ the parameter describing the rotational dependence of the torsional splitting is 0.9975. The implication for the energy level diagram of a value so close to 1 for this parameter will be discussed. Work is still in progress and it is hoped that it will be possible to identify torsional subbands in the crowded submillimeter wave spectrum recorded at room temperature

ANOMALOUS CENTRIFUGAL DISTORTION IN NH2

The ce{NH2} radical spectrum, first observed by Herzberg and_x000d_ Ramsay,footnote{Herzberg and Ramsay, {em J. Chem._x000d_ Phys.}~{bf 20} (1952) 347} is dominated by a strong_x000d_ Renner-Teller effectfootnote{Dressler and Ramsay, {em Phil._x000d_ Trans. R. Soc. A}~{bf 25} (1959) 553} giving rise to two_x000d_ electronic states: the bent $X, ^{2!}B_1$ ground state and_x000d_ the quasi-linear $A, ^{2!}A_1$ excited state. The ce{NH2}_x000d_ radical has been the subject of numerous high-resolution_x000d_ investigations and its electronic and ro-vibrational_x000d_ transitionsfootnote{label{data}Hadj Bachir, Huet,_x000d_ Destombes, and Vervloet, {em J. Molec. Spectrosc.}~{bf_x000d_ 193} (1999) 326; McKellar, Vervloet, Burkholder, and Howard,_x000d_ {em J. Molec. Spectrosc.}~{bf 142} (1990) 319; Morino_x000d_ and Kawaguchi, {em J. Molec. Spectrosc.}~{bf 182} (1997)_x000d_ 428} have been measured. Using synchrotron radiation, new_x000d_ rotational transitions have been recently recorded and a_x000d_ value of the rotational quantum number $N$ as large as 26_x000d_ could be reached.footnote{label{mam}Martin-Drumel, Pirali,_x000d_ and Vervloet, {em J. Phys. Chem. A}~{bf 118} (2014)_x000d_ 1331} In the $X, ^{2!}B_1$ ground state, the ce{NH2} radical_x000d_ behaves like a triatomic molecule displaying spin-rotation_x000d_ splittings. Due to the lightness of the molecule, a strong_x000d_ coupling between the overall rotation and the bending_x000d_ mode arises whose effects increase with $N$ and lead to_x000d_ the anomalous centrifugal distortion evidenced in the new_x000d_ measurements.$^d$_x000d_ _x000d_ In this talk the Bending-Rotation approachfootnote{Coudert,_x000d_ {em J. Molec. Spectrosc.}~{bf 165} (1994) 406} developed_x000d_ to account for the anomalous centrifugal distortion of the_x000d_ water molecule is modified to include spin-rotation_x000d_ coupling and applied to the fitting of high-resolution data_x000d_ pertaining to the ground electronic state of ce{NH2}._x000d_ A preliminary line position analysis of the available_x000d_ data$^{c,d}$ allowed us to account_x000d_ for 1681 transitions with a unitless standard deviation of 1.2._x000d_ New transitions could also be assigned in the spectrum recorded_x000d_ by Martin-Drumel {em et al.}$^d$_x000d_ In the talk, the results obtained with the new theoretical_x000d_ approach will be compared to those retrieved with a Watson-type_x000d_ Hamiltonian and the effects of the vibronic coupling between_x000d_ the ground $X, ^{2!}B_1$ and the excited $A, ^{2!}A_1$ electronic_x000d_ state will be discussed

EXPERIMENTAL AND COMPUTATIONAL INVESTIGATIONS OF THE THRESHOLD PHOTOELECTRON SPECTRUM OF THE HCCN RADICAL

The \ce{HCCN} radical, already detected in the interstellar_x000d_ medium, is also important for nitrile chemistry in Titan's_x000d_ atmosphere.\footnote{Gu\'elin and Cernicharo, {\em A\&A}~{\bf_x000d_ 244} (1991) L21; Loison {\em et al.,} {\em Icarus}~{\bf 247} (2015)_x000d_ 218} Quite recently the photoionization spectrum of the radical_x000d_ has been recorded\footnote{\label{spectrum}Garcia, Kr\"uger,_x000d_ Gans, Falvo, Coudert, and Loison, {\em J.\ Chem.\ Phys.}~(2017)_x000d_ submitted} using mass selected threshold photoelectron (TPE)_x000d_ spectroscopy and this provided us with the first spectroscopic_x000d_ information about the \ce{HCCN+} cation. Modeling such a_x000d_ spectrum requires accounting for the non-rigidity of \ce{HCCN}_x000d_ and for the Renner-Teller effect in \ce{HCCN+}._x000d_ _x000d_ In its $^3A''$ electronic ground state, HCCN is_x000d_ a non-rigid molecule as the potential_x000d_ for the \angle{\ce{HCC}} bending angle is very_x000d_ shallow.\footnote{Koput, {\em J.\ Phys.\ Chem.\ A}~{\bf 106}_x000d_ (2002) 6183} Vibronic couplings with the same_x000d_ bending angle leads, in the $^2\Pi$ electronic ground state of_x000d_ \ce{HCCN+}, to a strong Renner-Teller effect giving rise to a_x000d_ bent $^2A'$ and a quasi-linear $^2A''$ state.\footnote{Zhao,_x000d_ Zhang, and Sun, {\em J.\ Phys.\ Chem.\ A}~{\bf 112} (2008)_x000d_ 12125}_x000d_ _x000d_ In this paper the photoionization spectrum of the HCCN radical_x000d_ is simulated. The model developped treats the \angle{\ce{HCC}} bending_x000d_ angle as a large amplitude coordinate in both the radical_x000d_ and the cation and accounts for the overall rotation and_x000d_ the Renner-Teller couplings. Gaussian quadrature are used_x000d_ to calculate matrix elements of the three potential energy_x000d_ functions retrieved through {\em ab initio} calculations_x000d_ and rovibrational operators going to infinity for the linear_x000d_ configuration are treated rigorously._x000d_ _x000d_ The HCCN TPE spectrum is computed with the above model_x000d_ calculating all rotational components and choosing the_x000d_ appropriate lineshape. This synthetic spectrum will be_x000d_ shown in the paper and compared with the experimental_x000d_ one.$^b

MAGNETIC SPIN-TORSION COUPLING IN METHANOL

The hyperfine structure of non-rigid molecules in which hyperfine coupling arises from equivalent nuclei that can be exchanged by large amplitude motions is of great interest and lead to unexpected results. In the non-rigid (C$_2$D$_2$)$_2$ and (D$_2$O)$_2$ dimers, the hyperfine structure arising for nondegenerate tunneling sublevels can be accounted for using an effective quadrupole coupling Hamiltonian with the same coupling constant for all four deuterium atoms.footnote{Bhattacharjee, Muenter, and Coudert, {em J. Chem. Phys.}~{bf 97} (1992) 8850; and Stahl and Coudert, {em J. Mol. Spectrosc.}~{bf 157} (1993) 161.} In the non-rigid species CD$_3$COH and HCOOCH$_3$, the large amplitude torsional motion leads to hyperfine patterns which are qualitatively dependent on the torsional symmetry of the levels.footnote{Coudert and Lopez, {em J. Mol. Spectrosc.}~{bf 239} (2006) 135; and Tudorie, Coudert, Huet, Jegouso, and Sedes, {em J. Chem. Phys.}~{bf 134} (2011) 074314.} The interaction between a large amplitude torsional motion and the hyperfine coupling may also lead to a less known hyperfine effect, the so-called magnetic spin-torsion coupling, which was first studied by Heuvel and Dymanusfootnote{Heuvel and Dymanus, {em J. Mol. Spectrosc.}~{bf 45} (1973) 282 and {em ibid} {bf 47} (1973) 363.} and which has not yet been conclusively evidenced. In this talk, the magnetic hyperfine structure of the non-rigid methanol molecule will be investigated experimentally and theoretically. 13 hyperfine patterns were recorded using two molecular beam microwave spectrometers. These patterns, along with previously recorded ones,$^c$ were analyzed in an attempt to evidence the effects of the magnetic spin-torsion coupling. The theoretical approach setup to analyze the observed data accounts for the spin-torsion coupling, in addition to the familiar magnetic spin-rotation and spin-spin couplings, and relies on symmetry considerations to build a hyperfine coupling Hamiltonian and a spin-rotation-torsion wavefunction compatible with the Pauli exclusion principle. In the talk, the results of the analysis will be presented. The hyperfine coupling parameters retrieved will be discussed and we hope to be able to conclusively evidence the effects of the magnetic spin-torsion

Anomalous centrifugal distortion in HDO and spectroscopic data bases

The HDO molecule is important from the atmospheric point of view as it can be used to study the water cycle in the earth atmosphere.footnote{Herbin {em et al., Atmos. Chem. Phys.}~{bf 9} (2009) 9433; and Schneider and Hase, {em Atmos. Chem. Phys.}~{bf 11} (2011) 11207.} It is also interesting from the spectroscopic point of view as it displays an anomalous centrifugal distortion similar to that of the normal species H$_2$O. A model developed to treat the anomalous distortion in HDO should account for the fact that it lacks a two-fold axis of symmetry. A new treatment aimed at the calculation of the rovibrational energy of the HDO molecule and allowing for anomalous centrifugal distortion effects has been developed. It is based on an effective Hamiltonian in which the large amplitude bending $nu_2$ mode and the overall rotation of the molecule are treated simultaneously.footnote{Coudert, Wagner, Birk, Baranov, Lafferty, and Flaud, {em J. Molec. Spectrosc.}~{bf 251} (2008) 339.} Due to the lack of a two-fold axis of symmetry, this effective Hamiltonian contains terms arising from the non-diagonal component of the inertia tensor and from the Coriolis-coupling between the large amplitude bending $nu_2$ mode and the overall rotation of the molecule. This new treatment has been used to perform a line position analysis of a large body of infrared,footnote{Johns, {em J. Opt. Soc. Am. B}~{bf 2} (1985) 1340; Toth, {em J. Molec. Spectrosc.}~{bf 162} (1993) 20; Paso and Horneman, {em J. Opt. Soc. Am. B}~{bf 12} (1995) 1813; and Toth, {em J. Molec. Spectrosc.}~{bf 195} (1999) 73.} microwave,footnote{Messer, De Lucia, and Helminger, {em J. Molec. Spectrosc.}~{bf 105} (1984) 139; and Baskakov {em et al., Opt. Spectrosc.}~{bf 63} (1987) 1016.} and hot water vaporfootnote{Parekunnel {em et al., J. Molec. Spectrosc.}~{bf 210} (2001) 28; and Janca {em et al., J. Molec. Specrosc.}~{bf 219} (2003) 132.} data involving the ground and (010) states up to $J=22$. For these 4413 data, a unitless standard deviation of 1.1 was achieved. A line intensity analysis was also carried out and allowed us to reproduce the strength of 1316 transitions$^c$ with a unitless standard deviation of 1.1. In the talk, the new theoretical approach will be presented. The results of both analyses will be discussed and compared with those of a previous investigation.footnote{Tennyson {em et al., J. Quant. Spectrosc. Radiat. Transfer}~{bf 111} (2010) 2160.} The new spectroscopic data base built will be compared with HITRAN 2012.footnote{Rothman {em et al., J. Quant. Spectrosc. Radiat. Transfer}~{bf 130} (2013) 4.

EXPERIMENTAL AND THEORETICAL INVESTIGATIONS OF THE THRESHOLD PHOTOELECTRON SPECTRUM OF THE CH2 RADICAL

The methylene cation \ce{CH2+} is spectroscopically poorly characterized as it is difficult to produce in large amounts. It is subject to the Renner-Teller effect giving rise to ground \widetilde{X}^+\, ^2A_1 and excited \widetilde{A}^+\, ^2B_1 electronic states. Photoelectron spectroscopy of the methylene radical \ce{CH2} allows us to gain information about both \ce{CH2} and its cation. The former is also theoretically challenging as it is a very non-rigid species characterized by a barrier to linearity of less than 2000~cm$^{-1}$ in its ground \widetilde{X}\, ^3B_1 electronic state. The first photoelectron spectra of \ce{CH2} were investigated using pulsed-field-ionization zero-kinetic-energy spectroscopy.\footnote{\label{merkt}Willitsch {\em et al.,} {\em J.\ Chem.\ Phys.}~{\bf 117} (2002) 1939; and Willitsch \& Merkt, {\em ibid.}~{\bf 118} (2003) 2235} A rotationally resolved spectrum containing \widetilde{X}^+\, ^2A_1 \leftarrow \widetilde{X}\, ^3B_1 transitions was recorded from 83600 to 84070~cm$^{-1}$ and analyzed in terms of \ce{CH2+} rotational constants. The threshold photoelectron spectrum of \ce{CH2} has been recorded from 9.8 to 12~eV (79040 to 96800~cm$^{-1}$) using a recently developed flow tube reactor\footnote{Garcia {\em et al.,} {\em J. Chem. Phys.}~{\bf 142} (2015) 164201} and VUV synchrotron radiation. This new spectrum spans a larger energy range than the previous ones,\textsuperscript{$a$} but with less resolution. It displays narrow and broad features due respectively to the \widetilde{X}^+\, ^2A_1 \leftarrow \widetilde{X}\, ^3B_1 and \widetilde{A}^+\, ^2B_1 \leftarrow \widetilde{X}\, ^3B_1 ionizing transitions. Using new {\em ab initio} potential energy surfaces and available ones,\footnote{Jensen \& Bunker, {\em J.\ Chem.\ Phys.}~{\bf 89} (1988) 1327; and Jensen, Brumm, Kraemer \& Bunker, {\em J.\ Molec.\ Spectrsoc.}~{\bf 172} (1995) 194} the photoelectron spectrum is currently being computed using two models. The first one accounts for the large amplitude bending mode and the rotation only; the second one, also accounts for the stretching modes. The experimental and theoretical spectra will be discussed in the paper

Two-photon double ionization of neon using an intense attosecond pulse train

We present the first demonstration of two-photon double ionization of neon using an intense extreme ultraviolet (XUV) attosecond pulse train (APT) in a photon energy regime where both direct and sequential mechanisms are allowed. For an APT generated through high-order harmonic generation (HHG) in argon we achieve a total pulse energy close to 1 $\mu$J, a central energy of 35 eV and a total bandwidth of $\sim30$ eV. The APT is focused by broadband optics in a neon gas target to an intensity of $3\cdot10^{12}$W$\cdot$cm$^{-2}$. By tuning the photon energy across the threshold for the sequential process the double ionization signal can be turned on and off, indicating that the two-photon double ionization predominantly occurs through a sequential process. The demonstrated performance opens up possibilities for future XUV-XUV pump-probe experiments with attosecond temporal resolution in a photon energy range where it is possible to unravel the dynamics behind direct vs. sequential double ionization and the associated electron correlation effects