Laboratory rotational ground state transitions of NH3_3D+^+ and CF+^+

Aims. This paper reports accurate laboratory frequencies of the rotational ground state transitions of two astronomically relevant molecular ions, NH3D+ and CF+. Methods. Spectra in the millimeter-wave band were recorded by the method of rotational state-selective attachment of He-atoms to the molecular ions stored and cooled in a cryogenic ion trap held at 4 K. The lowest rotational transition in the A state (ortho state) of NH$_3$D$^+$ ($J_K = 1_0 - 0_0$), and the two hyperfine components of the ground state transition of CF$^+$($J = 1 - 0$) were measured with a relative precision better than $10^{-7}$. Results. For both target ions the experimental transition frequencies agree with recent observations of the same lines in different astronomical environments. In the case of NH$_3$D$^+$ the high-accuracy laboratory measurements lend support to its tentative identification in the interstellar medium. For CF$^+$ the experimentally determined hyperfine splitting confirms previous quantum-chemical calculations and the intrinsic spectroscopic nature of a double-peaked line profile observed in the $J = 1 - 0$ transition towards the Horsehead PDR.Comment: 7 pages, 2 figure

On the lowest ro-vibrational states of protonated methane: Experiment and analytical model

Protonated methane, CH$_5^+$, is the prototype of an extremely floppy molecule. To the best of our knowledge all barriers are surmountable in the rovibrational ground statethe large amount of zero-point vibrational energy leads to large amplitude motions for many degrees of freedom. Low resolution but broad band vibrational spectroscopy [1] revealed an extremely wide range of C-H stretching vibrations. Comparison with theoretical IR spectra supported the structural motif of a CH$_3$ tripod and an H$_2$ moiety, bound to the central carbon atom by a 3c2e bond. In a more dynamic picture the five protons surround the central carbon atom without significant restrictions on the H-C-H bending or H$_n$-C torsional motions. The large-amplitude internal motions preclude a simple theoretical description of the type possible for more conventional molecules, such as the related spherical-top methane molecule. Recent high-resolution ro-vibrational spectra obtained in cold ion trap experiments [2] show that the observed CH$_5^+$ transitions belong to a very well-defined energy level scheme describing the lowest rotational and vibrational states of this enigmatic molecule. Here we analyse the experimental ground state combination differences and associate them with the motional states of CH$_5^+$ allowed by Fermi-Dirac statistics. A model Hamiltonian for unrestricted internal rotations in CH$_5^+$ yields a simple analytical expression for the energy eigenvalues, expressed in terms of new quantum numbers describing the free internal rotation. These results are compared to the experimental combination differences and the validity of the model will be discussed together with the underlying assumptions. \newline [1] O. Asvany, P. Kumar, I. Hegemann, B. Redlich, S. Schlemmer and D. Marx, Science {\bf 309}, (2005) 1219-1222 \newline [2] O. Asvany, K.M.T. Yamada, S. Br\"{u}nken, A. Potapov, S. Schlemmer, Science {\bf 347} (2015) 1346-134

SPECTROSCOPIC STUDIES OF PROTONATED AMINES: CH3NH3+ AND C2H5NH3+

Mid-infrared spectra of protonated methylamine, CH$_3$NH$_3^+$, and ethylamine, C$_2$H$_5$NH$_3^+$, have been recorded using the FELion ion trap connected to the Free Electron Laser for Infrared eXperiments (FELIX; Radboud University, Nijmegen, The Netherlands) employing infrared photodissociation of the corresponding neon-clusters. In addition, the pure rotational spectrum of CH$_3$NH$_3^+$ has been observed for the first time. Rotational transitions were observed in the frequency region between 80 and 240\,GHz in the Coltrap apparatus using the method of state-selective He-attachment. In contrast to methylamine which features a complex CH$_3$-internal-rotation/NH$_2$-inversion spectrum, its protonated variant CH$_3$NH$_3^+$ exhibits the spectrum of a simple symmetric rotor in its ground vibrational state

HIGH RESOLUTION SPECTROSCOPY OF 3-METHYLBUTYRONITRILE BETWEEN 2 AND 400 GHZ

We present high-resolution rotational spectroscopy of the two conformers of 3-methylbutyronitrile. Spectra were taken between 2-24~GHz by means of Fourier transform microwave spectroscopy. Spectra between 36 and 402~GHz were recorded by means of frequency modulated (FM) absorption spectroscopy. The analysis yields precise rotational constants and higher order distortion constants, as well as a set of $^{14}$N nuclear electric quadrupole coupling parameters. In addition, quantum chemical calculations were performed assisting the assignments. Frequency calculations yield insight into the vibrational energy structure from which partition functions and vibrational correction factors are determined. These are used to determine experimentally and computationally the energy difference between the conformers, which is revealed to be negligeable. Overall, this study provides precise spectroscopic constants for the search of 3-methylbutyronitrile in the interstellar medium. In particular, this molecule is an interesting testcase to check our knowledge of (branched) molecule formation in space

HIGH-RESOLUTION SPECTROSCOPY OF TWO CONFORMERS OF 2-CYANOBUTANE BETWEEN 10 AND 400 GHZ

We present high-resolution rotational spectroscopy of two out of three conformers of 2-cyanobutane. Spectra were taken between 10-26 GHz by means of chirped-pulse spectroscopy. Spectra between 36 and 402 GHz were recorded by means of frequency modulated (FM) absorption spectroscopy. The analysis yields precise rotational constants and higher order distortion constants, as well as a set of $^{14}$N nuclear electric quadrupole coupling parameters. In addition, quantum chemical calculations were performed assisting the assignments. Calculations of vibrational frequencies yield insight into the vibrational energy structure from which partition functions and vibrational correction factors are determined. These are used to determine experimentally and computationally the energy difference between the conformers. Overall, this study provides precise spectroscopic constants for the search of 2-cyanobutane in the interstellar medium. In particular, this molecule appears as an interesting case to test our knowledge of (branched) molecule formation in space

ROTATIONAL ACTION SPECTROSCOPY VIA STATE-SELECTIVE HELIUM ATTACHMENT

Helium atoms can attach to molecular cations via ternary collision processes forming weakly bound ($approx 1$ kcal/mol) He-M$^+$ complexes. We developed a novel sensitive action spectroscopic scheme for molecular ions based on an observed rotational state dependency of the He attachment process [1]. A detailed account of the underlying kinetics will be presented on the example of the CD$^+$ ion, where our studies indicate a decrease of around 50% for the rotational state dependent ternary He attachment rate coefficient of the $J=1$ level with respect to the $J=0$ level. Experiments are performed on mass-selected ions stored in a temperature-variable ($Tgeq 3.9$~K) cryogenic rf 22-pole ion trap in the presence of a high number density of He ($approx 10^{15}$ cm$^{-3}$) [2]. Rotational spectra of the bare ions are recorded by measuring the change in the number of formed He-M$^+$ complexes after a certain storage time as a function of excitation wavelength. Here we will also present the first measurements of the rotational ground state transitions of CF$^+$ ($J=1-0$, hfs resolved) and NH$_3$D$^+$ ($J_K=1_0-0_0$), recorded in this way.\ noindent [1] Br\"{u}nken et al., ApJL {bf 783}, L4 (2014) \ noindent [2] Asvany et al., Applied Physics B {bf 114}, 203 (2014

STATE-TO-STATE ROTATIONAL RATE COEFFICIENTS FOR AMMONIA SELF COLLISIONS FROM PUMP-PROBE CHIRPED-PULSE EXPERIMENTS

Rotational state populations of ammonia are inferred from chirped pulse spectra of its tunneling doublets in a room temperature K-band waveguide experiment where many tunneling doublets can be addressed by a single chirped pulse excitation. The thermal distribution of states is altered by a pump pulse where the population of the tunneling doublet of a single rotational state is inverted by a $\pi$-pulse within roughly 100 ns. The resulting deviation from equilibrium is then propagating to other states due to collisions and interrogated by a probe pulse from which the state populations of many rotational states are inferred at once. From the free induction decays (FID) of the individual states the relaxation time of the radiation-induced superposition state of the two level tunneling system (T2) is inferred. Also the collisional relaxation time (T1) for the difference in the population of the two-level system is determined. These values exhibit a linear pressure dependence, the slope of which agrees very well with previous measurements % \footnote{P. E. Wagner et al 1981 J. Phys. B: At. Mol. Phys., vol. 14, 4763.}. % \\ Analysis of probe FID signals from these pump-probe experiments reveals the well known hierachy of collisional relaxation in ammonia which was first found by Oka fifty years ago through steady state intensity measurements % \footnote{T. Oka, 1968, J.Chem.Phys., vol 48, 4919}. % Collision-induced transitions within the tunneling doublet ($\Delta J$ = 0) determined from T1 measurements are faster than $\Delta J$ = $\pm$1 transitions. Of those the $\Delta K$ = 0 transitions are much faster than those with $\Delta K \ne$ 0. Due to this hierachy of inelastic processes and thanks to the fast optical pumping experiments state-to-state rates can be measured. As a result, from the pressure dependence of the measured rates state-to-state rate coefficients are determined. Those rate coefficients agree very well with results of simulations of all coupled states which fit with the temporal behavior of the complete pump probe experiments where many individual (J,K) rotational states can be addressed step by step by separate probe-pump-probe pulse sequences

HIGH RESOLUTION ROTATIONAL SPECTROSCOPY OF CH3+-He

Using an infrared and millimeter double resonance action spectroscopic scheme in a cryogenic ion trap, high resolution rotational transitions are recorded for the (near) symmetric top CH$_3^+$-He. Eighteen rotational transitions up to $J''=6$ are recorded in the range 60 - 410~GHz. Small unexpected splittings are resolved for $K=1$. Advantages of this novel approach of high-resolution ion spectroscopy and potential future applications to spectra of cold cation-helium complexes are discussed