Photodissociation dynamics of propargylene, HCCCH

Abstract

We report a joint theoretical and experimental study on the photodissociation of the C3H2 isomer propargylene, HCCCH, combining velocity map imaging with nonadiabatic trajectory surface hopping calculations. Propargylene loses an H-atom, after laser excitation at around 250 nm, presumably to the T6 state. The photofragment angular distribution exhibits only a very small anisotropy, but the H-atom translational energy distribution extends to high energies and shows an expectation value of 〈fT〉, the fraction of excess energy released as translation, of 48%, outside the range expected for a statistical reaction mechanism. The computations suggest a predissociation in the T4-T6 state and lead to a translational energy distribution and photofragment angular distribution that match the experimentally observed ones very well.

Photodissociation Dynamics of Cyclopropenylidene, c‐C3H2

“…Based on RRKM calculations we assume a branching ratio of 85:15 in favour of c ‐C 3 H formation. Furthermore rotational energy has to be included into the product energy distribution, because pyrolytically generated species often possess a rotational temperature of around 150 K 7. 27 With these assumptions the translational energy distribution was fitted by a function close to a prior distribution, which confirms that the reaction is statistical in nature.…”

“…However, as visible the distribution decays to zero at 0.3 eV TER. There are two possible reasons: First, the pyrolysis might produce vibrationally excited carbenes, given the typical vibrational temperatures of 150–200 K 7. Second, multiphoton processes might contribute.…”

“…As visible the fit is not a smooth function, but shows step‐like features that are associated with the onset of product vibrational states. These steps are washed out in the experiment most likely due to the rotational temperature that is assumed to be around 150 K 7. 27…”

“…However,a sv isible the distribution decayst oz ero at 0.3 eV TER. Therea re two possible reasons: First, the pyrolysis might produce vibrationally excited carbenes, given the typical vibrational temperatures of 150-200 K. [7] Second, multiphoton pro- www.chemeurj.org cesses might contribute. In fact, when the TER wasr ecorded at twice the excitation laser power, the distribution extended to even highere nergies, while the maximum of the distribution did not change.…”

“…[5] All three isomersa re also relevantf or combustion chemistry and have been detected in flames. [6] Propargylenew as recently studied by velocitym ap imaging [7] and by negative-ionphotoelectron spectroscopy. [8] The astrochemical importance of c-C 3 H 2 triggered laboratory work by microwave spectroscopy, [9] matrix isolation IR spectroscopy [10] and photoionization.…”