Electronic absorption spectra of the protonated polyacetylenes HC2nH2+ (n=3,4) in the gas phase

Abstract

A new approach has been developed for the purpose of measuring the electronic transitions to bound exited states for cations that have been collisionally relaxed to low vibrational and rotational temperatures. This has been used to obtain the first gas phase electronic spectra of the protonated polyacetylenes using a two-color ion-photodissociation approach. Specifically, the origin bands in the B (1)A(1)<-- X(1)A(1) transitions of HC(6)H(2) (+) and HC(8)H(2) (+) (C(2v) geometry) were observed at 26,403.3 and 21,399.8 cm(-1). Data on such cooled systems allow a direct comparison between laboratory and astrophysical measurements.

Electronic spectra of C6H+ and C6H3+ in the gas phase

“…This was achieved here with the cavity ringdown approach. However, the structure could still not be resolved and the same rotational profile as shown in Figure 3 in reference [9] was observed. Thus, it is concluded that the excited electronic state is short-lived as a result of lifetime broadening by intramolecular processes.…”

“…An electronic transition of mass-selected C 6 H 3 ϩ could already be observed in the gas phase by using iontrapping technology [9]. However the possibility of laser power saturation in such experiments precluded a conclusion whether the excited-state structure was lifetime broadened or the resolution was just inadequate.…”

“…This was subsequently detected at 26402 cm Ϫ1 in the gas phase in an ion trap [9]. A two-color photon scheme was used to observe this; a bound-bound transition out of the ground electronic state was excited with one laser while the second UV laser subsequently produced the fragment ion C 6 H 3 ϩ , which was monitored.…”

“…Their resolution would prove the structure of the species; either the "classical" HC 6 H 2 ϩ with C 2v symmetry, or the nonclassical linear HC 6 H form with a loosely bound H ϩ undergoing large amplitude motion. According to ab initio calculations [9], the electronic transition observed could correspond to 1 A 1 Ϫ X 1 A 1 in C 2v symmetry. On the basis of the infrared frequencies observed in measurements on mass-selected C 6 H 3 ϩ in neon-matrices [18], the classical H 2 C 6 H ϩ structure was also implied.…”

Perspective: C60+ and laboratory spectroscopy related to diffuse interstellar bands

“…24 A schematic of the current ion trapping apparatus is shown in Figure 3. In this approach 10 3 -10 4 buffer gas cooled ions are held in a radiofrequency (rf) trap where one tunable laser is used to search for the electronic transition of interest and a second fixed frequency laser of higher energy is used to fragment the electronically excited ions.…”