The A 3Σ−–X 3Σ− electronic transition of HC6N

Abstract

A combined matrix and gas phase study is presented to identify the A 3 ⌺ Ϫ -X 3 ⌺ Ϫ electronic transition of the linear triplet isomer of HC 6 N and isotopic derivative DC 6 N. Absorption spectra have been observed in a 6 K neon matrix after mass selective deposition and in the gas phase by cavity ring down spectroscopy through a supersonic planar plasma. The band origin of the 0 0 0 A 3 ⌺ Ϫ -X 3 ⌺ Ϫ electronic transition of HC 6 N is determined to be at 21 208.60͑5͒ cm Ϫ1 , shifted ϳ30 cm Ϫ1 to the blue of the neon matrix value. Rotational analysis indicates that the chain is slightly stretched on electronic excitation, yielding B 0 Јϭ0.027 92(5) cm Ϫ1 . Transitions to vibrationally excited levels in the upper A 3 ⌺ Ϫ state are observed as well. The results are compared with a rotationally resolved spectrum of the 0 0 0 A 3 ⌺ u Ϫ -X 3 ⌺ g Ϫ electronic transition of the isoelectronic HC 7 H species.

Rotationally resolvedA3Σu-–

“…The observed spectrum resembles that of the iso-electronic cyanopolyyne HC 6 N [14] and dicyanopolyyne, C 5 N 2 [20]. Both these molecules have been studied previously in a similar way, also combining cavity ring-down spectroscopy and supersonic planar plasma expansions.…”

“…The (fitted) band origin is located at 19817.892(1) cm À1 , which is about 0.1 cm À1 (0.03 cm À1 ) red shifted from the value reported in Ref. [13] ( [14]). This is well within the reported uncertainties with which the absolute frequencies could be determined.…”

“…The use of a planar plasma source (as in Ref. [14]), providing an essentially Doppler-free environment, instead of a pinhole nozzle, and an improvement in laser bandwidth by operating a commercial pulsed dye laser system in a higher grating order, allows for recording nearly forty subsequent transitions in the A 3 R À u -X 3 R À g origin band spectrum around 504.5 nm.…”

“…The matrix bands are broadened and their absorption frequencies are typically shifted by as much as 100 cm À1 compared to the gas phase -reason why these data cannot be used to compare directly with DIB positions -but as such they are indicative for where to start searching in the gas phase. Using the available matrix data, electronic spectra of HC 7 H, HC 9 H, HC 11 H and HC 13 H have been recorded in the gas phase [12][13][14]. In the case of HC 7 H the matrix shift for the origin band turned out to be only 7 cm À1 .…”

“…In Ref. [14] a better resolved spectrum was achieved, but only a contour fit was presented based upon a theoretically predicted ground state B-value (taking B 0 B e ). The present study extends the spectroscopic characterization of the A 3 R À u -X 3 R À g origin band transition of HC 7 H and provides a full rotational analysis.…”