13 A Spectroscopic Survey of Electronic Transitions of C6H, C6H, and C6D X. Bacallaa,b, E.J. Salumbidesa,c, H. Linnartzb, W. Ubachsa, D. Zhaoa,b,1,˚ aDepartment of Physics and Astronomy, LaserLaB, VU University Amsterdam, De Boelelaan 1081, NL-1081 HV Amsterdam, The Netherlands bSackler Laboratory for Astrophysics, Leiden Observatory, Leiden University, PO Box 9513, NL-2300 RA Leiden, The Netherlands cDepartment of Physics, University of San Carlos, Nasipit, Talamban, Cebu City 6000, Philippines Abstract ´1 Electronic spectra of C6H are measured in the 18 950 ´ 21 100 cm domain using cavity ring-down spectroscopy of a supersonically expanding hydrocarbon plasma. In total, 19 (sub)bands of C6H are presented, all probing the vibrational manifold of the B2Π electronically excited state. The assignments are guided by electronic spectra 13 available from matrix isolation work, isotopic substitution experiments (yielding also spectra for C6H and C6D), predictions from ab initio calculations as well as rotational fitting and vibrational contour simulations using the 0 available ground state parameters as obtained from microwave experiments. Besides the 00 origin band, three non-degenerate stretching vibrations along the linear backbone of the C6H molecule are assigned: the ν6 mode associated with the C–C bond vibration and the ν4 and ν3 modes associated with C”C triple bonds. For the two 2 2 lowest ν11 and ν10 bending modes, a Renner-Teller analysis is performed identifying the µ Σ(ν11) and both µ Σ(ν10) 2 and κ Σ(ν10) components. In addition, two higher lying bending modes are observed, which are tentatively assigned 2 2 2 2 as µ Σ(ν9) and µ Σ(ν8) levels. In the excitation region below the first non-degenerate vibration (ν6), some Π ´ Π transitions are observed that are assigned as even combination modes of low-lying bending vibrations. The same 2 2 holds for a Π ´ Π transition found above the ν6 level. From these spectroscopic data and the vibronic analysis a 2 comprehensive energy level diagram for the B Π state of C6H is derived and presented. Keywords: hexatriynyl radical, cavity ring-down spectroscopy, isotopic substitution, supersonic plasma discharge Introduction guided two gas phase studies. 6,16 An unresolved elec- tronic origin band as well as several hot bands were The hexatriynyl radical, C6H, belongs to the best recorded for a rather high temperature (Trot „ 150 K) studied linear carbon chain radicals. As several other using a liquid-nitrogen cooled hollow cathode dis- 16 members in the acetylenic C2nH series, it has been charge. After this, a study of a supersonically expand- identified in different environments in the interstel- ing and adiabatically cooled plasma resulted in a full lar medium. 1–4 Microwave spectroscopic work 5 has pro- rotational analysis of the two spin-orbit components of 2 2 2 vided accurate spectroscopic constants for the X Π elec- the Π´X Π origin band spectrum for both C6H and 6 tronic ground state that later were extended to deuter- C6D radicals. An extended study, presenting more ac- 6 ated hexatriynyl (C6D) as well as to vibrationally ex- curate data for the electronic origin band as well as a 7 8–10 1 2 2 cited C6H. Ab initio calculations, together with in- Renner-Teller analysis for the t11u1 µ Σ ´ µ Σ vibronic frared (IR) work, 11 have also been conducted to pre- hot band, was presented several years later. 17 In the op- 2 dict and identify the fundamental vibrational frequen- tical spectra only the Σ vibronic component of the ν11 2 cies of C6H. Recently, the astronomical detection of vi- mode was observed, in contrast to the ∆ component, 7,12 7 brationally excited C6H and the chemically related which was detected in the microwave spectrum, but – 13 ´1 C6H anion triggered a renewed spectroscopic inter- which is most likely too energetic („ 100 cm ) to be observed in cold jet expansions. The same holds for the arXiv:1608.00880v1 [physics.chem-ph] 2 Aug 2016 est in this molecule. low-lying electronic state with 2Σ symmetry, at an exci- The first electronic spectra of C6H were recorded us- ing matrix isolation spectroscopy; 14,15 results of which tation energy of „ 1 774 cm´1, which was observed in a photo-detachment study. 18 These states were predicted in ab initio electronic structure calculations. 10,19 Har- ˚ Corresponding author. monic vibrational frequencies of the various modes in Email address: [email protected] (D. Zhao) C6H were ab initio calculated, yielding the lowest mode, 1 Present address: Hefei National Laboratory for Physical Sciences ´1 ν11, at some 100 cm and the highest mode at about at the Microscale, Department of Chemical Physics, University of ´1 9 Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 3 300 cm . Also of interest is the spectroscopic inves- + 20 230026, P. R. China. tigation of the C6H cation, both in a neon matrix and Accepted for publication in The Journal of Physical Chemistry A 26 July 2016 The Journal of Physical Chemistry A Accepted Article in the gas phase. 21 0.090 cm´1. Wavelength calibration is achieved by In the present work, cavity ring-down spectroscopy recording simultaneously an etalon transmission spec- of a supersonically expanding hydrocarbon plasma has trum (FSR „ 20.1 GHz) which provides relative fre- been employed in a wider-ranging spectroscopic survey. quency markers for correction of any nonlinearity dur- All spectroscopic features observed are associated with ing scanning. An iodine (I2) spectrum is recorded for the B2Π electronically excited state and its vibrational absolute wavelength calibration, yielding an overall ac- ´1 ´1 modes. From the C6H origin band at 18 980 cm to- curacy of better than 0.02 cm . wards higher wavenumbers, several new spectral fea- The use of the slit nozzle leads to nearly Doppler-free tures have been observed and found to originate from spectra due to a collimation effect on the gas pulses 13 C6H as well. Also, a substantial number of C6H (full emanating from the slit jet expansion perpendicular 13 C substitution) and C6D bands has been observed for to the laser beam inside the ring-down cavity. In the the first time. case of the pinhole nozzle the contribution of molecules with velocity components along the laser beam path is larger, therewith producing spectral lines with ex- Experiment tended Doppler broadening. However, the latter ef- fect can be partially counteracted by mixing Ar in the The experimental setup and the measurement proce- gas sample, which produces a favorable effect on the dure are similar to those described before. 17,22 The car- spectral linewidth. This advantage is exploited for the 13 bon chain radicals are formed by discharging different recording of C6H spectra. The isotopically enriched 13 gas mixtures in the throat of a pulsed plasma nozzle, C2H2 was used in combination with the pinhole noz- 22 23 either a slit nozzle or pinhole design. For C6H and zle which uses up less gas than the slit nozzle. A few 12 12 13 C6D, 0.5% C2H2 in He and 0.3% C2D2 in He mix- of the C-containing bands were nevertheless recorded tures are used, respectively. An isotopically enriched in the slit nozzle configuration. 13 C2H2 (99% purity) gas sample, mixed at 0.13–0.18% Obviously, the optical methods used are not mass se- in He/Ar (85/15) is used to record fully 13C-substituted lective, and spectra contain information from a num- 13 C6H spectra. A solenoid valve (General Valve, Series ber of species formed in the plasma, including separate 9) on top of the nozzle body controls a high pressure transitions originating from, for example, C2,C3, or CH. (5–10 bar) gas pulse (roughly of 1 ms duration) that is Also, under similar experimental conditions, long lin- discharged between ceramically isolated electrodes be- ear 25 and bent chain 26,27 carbon-based molecules are fore expanding into a vacuum, generated by a powerful formed in the plasma expansion. While under the roots blower system. Typically, 300 to 500 µs long neg- presently used plasma expansion conditions the larger ative high voltage pulses (V/I „ ´750 V / 100 mA) are molecules are rotationally and vibrationally cooled, typ- offered to dissociate the precursor species and to allow ically exhibiting rotational temperatures of Trot „ 20 K molecule formation through collisions in the expanding and vibrational temperatures of the same values, 17 plasma. In parallel the adiabatic expansion allows for the diatomic radicals C2 and CH are detected at ele- final rotational temperatures in the order of 15 to 25 K. vated temperatures even allowing the detection of C2 3 25 The output of a pulsed Nd:YAG-pumped dye laser is in the metastable a Πu state in the v “ 9 level. It aligned just below (2–5 mm) and parallel to a 3 cm ˆ is noted that the resulting rotational temperature can 300 µm slit nozzle, or across the conical jet of a 1.0 mm be regulated by changing the nozzle-to-laser-beam dis- diameter pinhole. The effective absorption pathlength tance. 17,23 Further, the plasma chemistry is different along the slit or through the pinhole expansion can be under conditions of a conical expansion when a pinhole further improved by orders of magnitude by position- nozzle is employed, or for a planar expansion in the ing the slit nozzle (or pinhole) along the optical axis case of a slit jet configuration. The spectral features between two highly reflective mirrors (R > 0.99998) in associated with C2 and CH, which in some cases can a cavity ring-down configuration.