Photodissociation of propyne and allene at 193 nm with vacuum ultraviolet detection of the products

Abstract

Vacuum ultraviolet ͑VUV͒ laser photoionization is combined with time-of-flight ͑TOF͒ mass spectrometry to determine the photofragments produced from the laser photodissociation of allene and propyne in a molecular beam. Detection of C 3 H 3 ϩ confirms that atomic hydrogen elimination is the primary process for both of these molecules. A hydrogen molecule elimination channel and a low mass carbon fragmentation channel of allene to produce C 3 H 2 ϩH 2 and CH 2 ϩC 2 H 2 , respectively, have also been identified. Different ratios of various dissociation channels from these two molecules suggest that the dissociation mechanisms of these two isomers are different. Dissociation must occur before complete isomerization. These results are discussed in terms of recent theoretical calculations on the ground and excited states of these molecules. Secondary photodissociation of the products has been observed, even though the laser energies that have been used are less than 8 mJ/cm 2 and the photolysis laser is not focused. Therefore, the present results show how important it is to determine product distributions as a function of the laser energy.

Chemistry of Energetically Activated Cumulenes—From Allene (H2CCCH2) to Hexapentaene (H2CCCCCCH2)

“…The C 3 H 2 fragments produced both from allene and propyne were assigned as vinylidenecarbene (H 2 CCC), a small amount of which was also observed from secondary dissociation of C 3 H 3 . The C 3 H 3 + H/ C 3 H 2 + H 2 branching ratios were evaluated as 64:36 and 56:44 for allene and propyne, respectively, which is at odds with the previous measurements by Jackson et al [97] and Ni et al [101] Four years later, DeSain and Taatjes [104] studied 193 nm photolysis of propyne using infrared laser absorption spectroscopy and reported even lower quantum yield of the C 3 H 3 radical, 0.49 AE 0.1, though they disagreed on the identity of the C 3 H 3 isomer produced, which, based on the observed infrared spectra, they concluded to be the propargyl radical. Interestingly, they also deduced that acetylene is a primary product of propyne photolysis, with a quantum yield of about 0.1.…”

“…Nevertheless, the later paper from Neumark's group in 2005 [107] fundamentally agrees with the results of Qadiri et al in that the H(D) loss processes from allene, propyne, and D3-propyne are preceded by isomerization and statistical equilibration. However, subtle differences were still observed for the other dissociation channels (H 2 , D 2 , and HD loss in the Neumark's study [107] and the observation of a trace CH 2 + C 2 H 2 channel from allene in the work of Ni et al [101] ), indicating that there is no full equilibration between allene and propyne before decomposition.…”

“…The H/H/CH 2 -branching ratios were evaluated as 1:0.15:0.27 indicating that the increase of the photon energy and consequently the internal energy available to allene strongly favor the CH 2 + C 2 H 2 and C 3 H 2 + H 2 channels against the atomic hydrogen; recall that Ni et al evaluated the branching ratios as 1:0.075:0.01 at 193 nm. [101] Harich et al also studied the photodissociation of propyne at 157 nm, including its deuterated counterparts CD 3 CCH and CH 3 CCD. [109] The hydrogen atom loss was observed both from the methyl group and from the ethynyl radical site.…”

Methylacetylene (CH3CCH) and propene (C3H6) formation in cold dense clouds: A case of dust grain chemistry

“…86.7 % de C 3 H x (supposed to be C 3 H 4 **) and 13.3% of CH x + C 2 H y (Angelova et al 2004a, Angelova et al 2004b. We supposed than C 3 H 4 ** leads mainly to C 3 H 3 by comparison with C 3 H 4 photodissociation (Seki & Okabe 1992, Ni et al 1999, Harich et al 2000a, Harich et al 2000b. (Faravelli et al 2000, Miller et al 2008 Figure 1 Abundances of methylacetylene (continuous line) and propene (dashed line) as a function of time predicted by our models with n(H 2 ) = 2 ⋅ 10 4 cm -3 using a model with gas phase reactions only.…”

Photochemistry of C3Hphydrocarbons in Titan’s stratosphere revisited

“…Photolysis studies of these molecules have been performed at 193 nm (Seki & Okabe 1992;Ni et al 1999) and at 155 nm (Harich et al 2000a,b). There are some differences.…”