Formation of Metastable Triplet Acetylene from the Ã(¹A_u) State Near the Dissociation Threshold

Abstract

The triplet metastable states of acetylene produced by intersystem crossing from the Ã(1Au) state have been detected by a sensitized phosphorescence (SP) method. The phosphorescence was observed from C2H2 in the energy region below the barrier to dissociation in the ã state suggested by Hashimoto and Suzuki [J. Chem. Phys. 1996, 104, 6070]. The lifetimes of the triplet states coupled with the V3K and V4K1 levels in the Ã(1Au) state were estimated to be 80 and 100 μs. The rotational structures of the laser-induced fluorescence and SP spectra were similar, except that the enhancement of many weak absorption lines makes the latter more congested. The feature of the SP spectrum exemplifies the complicated singlet−triplet mixing at the V3K1 level. The SP signal was observed with different phosphors, where benzil was found to be the most sensitive. The SP spectrum of C2D2 was observed only by using benzil. In addition to SP, the collision of metastable triplet acetylene with a phosphor surface yielded fluorescence. This is most likely to be fluorescence from the Ã(1Au) state of acetylene. The lifetime of the triplet states and the threshold energy to dissociation in C2H2 and C2D2 suggest that tunneling occurs on the ã state and that the energy barrier to dissociation is higher than the previous estimation.

Predissociation of acetylene from the à 1Au state studied by absorption, laser-induced fluorescence, and H-atom action spectroscopies

“…It is noted that similar fall-off of fluorescence occurs between the 1 1 2 1 V 2 K 1 and 1 2 V 1 K 1 levels in C 2 D 2 , 6,20 which means that the fall-off of fluorescence in C 2 D 2 occurs in higher energy than C 2 H 2 by 960 cm Ϫ1 . Normally, an isotope effect arises from the difference of the zero point energy between the isotopomers.…”

“…From the fluorescence decay of C 2 D 2 , the potential barrier is estimated to be 750-900 cm Ϫ1 . 20 The V 4 K 1 level is located below the fluorescence fall-off energy and its photophysical quantities are similar to those of V 3 K 1 . Nevertheless, the H atom signal is formed from this level.…”

“…Sensitized phosphorescence spectroscopy has shown that metastable triplet states with the lifetime of 80 s are produced from this level. 20 The results indicate that dissociation from V 4 K 1 occurs from the ground state.…”

“…18͒ studies performed below the dissociation threshold. Shi and Suzuki have detected the metastable triplet molecules produced from the à state using molecular beam sensitized phosphorescence method, 19,20 which provided direct evidence for intersystem crossing. Humphrey et al have reported similar work by the Auger method.…”

Spectroscopy and energetics of the acetylene molecule: dynamical complexity alongside structural simplicity

“…There is an extensive body of literature -both experimental [183,185,[188][189][190][191][192][193][194][195] and theoretical [196][197][198][199] -on singlet-triplet interactions in C 2 H 2 , with intersystem crossing from triplet (S ¼ 1) states yielding channels for à -state C 2 H 2 to predissociate into C 2 H (X 2 AE þ ) þ H. Two pairs of cis-and trans-bent isomers have been identified: a˜3B 2 andb 3 B u , andc 3 A u andd 3 A 2 , in order of increasing energy. These lie above 28 500 cm À1 [197,198] and are therefore inaccessible to either one-or two-photon excitation within the S 0 linearX 1 AE þ g electronic ground state of C 2 H 2 by IR PUMP radiation used in IR-UV DR experiments that as discussed in section 5.2.4 and 5.2.5 below.…”