Laser induced fluorescence detection and characterization of ionic and metastable reaction products

Fluorescence-dip infrared spectroscopy ͑FDIRS͒ is employed to record the infrared spectra of the isolated, jet-cooled tropolone molecule ͑TrOH͒ and its singly deuterated isotopomer TrOD in the O-H and C-H stretch regions. The ability of the method to monitor a single ground-state level enables the acquisition of spectra out of the lower and upper levels of the zero-point tunneling doublet free from interference from one another. The high power of the optical parametric oscillator used for infrared generation produces FDIR spectra with good signal-to-noise despite the weak intensity of the C-H and O-H stretch transitions in tropolone. The expectation that both spectra will exhibit two OH stretch transitions separated by the OH(vϭ1) tunneling splitting is only partially verified in the present study. The spectra of TrOH are compared with those from deuterated tropolone ͑TrOD͒ to assign transitions due to C-H and O-H, which are in close proximity in TrOH. The appearance of the spectra out of lower ͑a 1 symmetry͒ and upper ͑b 2 symmetry͒ tunneling levels are surprisingly similar. Two sharp transitions at 3134.9 cm Ϫ1 ͑out of the a 1 tunneling level͒ and 3133.9 cm Ϫ1 ͑out of the b 2 tunneling level͒ are separated by the ground-state tunneling splitting ͑0.99 cm Ϫ1 ͒, and thereby terminate in the same upper state tunneling level. Their similar intensities relative to the C-H stretch transitions indicate that the y-and z-polarized transitions are of comparable intensity, as predicted by ab initio calculations. The corresponding transitions to the other member of the upper state tunneling doublet are not clearly assigned by the present study, but the broad absorptions centered about 12 cm Ϫ1 below the assigned transitions are suggested as the most likely possibility for the missing transitions.

The photochemistry of diacetylene (C4H2), the largest hydrocarbon to be unambiguously identified in planetary atmospheres, is of considerable importance to understanding the mechanisms by which complex molecules are formed in the solar system. In this work, the primary products of C4H2's ultraviolet photochemistry were determined in a two-laser pump-probe scheme in which the products of C4H2 photoexcitation are detected by vacuum ultraviolet photoionization in a time-of-flight mass spectrometer. Three larger hydrocarbon primary products were observed with good yield in the C4H2* + C4H2 reaction: C6H2, C812, and C8H3. Neither C6H2 nor C8H3 is anticipated by current photochemical models of the atmospheres of Titan, Uranus, Neptune, Pluto, and Triton. The free hydrogen atoms that are released during the formation of the C8H3 and C8H2 products also may partially offset the role of C4H2 in catalysing the recombination of free hydrogen atoms in the planetary atmospheres.

The collisional photochemistry of diacetylene triplet metastable state (C4H2*) with acetylene in nitrogen and helium buffers is studied in a reaction tube attached to a pulsed nozzle. The photolysis laser counterpropagates the molecular expansion through the reaction tube exciting the C4H2 'Au -I&+ 2'06'0 transition at 231.4 nm. Efficient intersystem crossing forms the triplet state from which reaction occurs. The finite 8 mm length of the tube serves to quench the reaction after 10-30 ps so that primary products and not polymer are formed. Upon exiting the reaction tube, the photochemical products are soft ionized with 118 nm vacuumultraviolet light and mass analyzed in a linear time-of-flight mass spectrometer. The primary photochemical processes are C4H2* + C2Hz -C6H2 + 2H (H2) and C4H2* + C2H2 -C6H3 -I-H. Under all conditions examined, C6H2 dominates the observed products, with C6H3 present above the I3CCsH2 background only at high nitrogen concentrations which deactivate diacetylene within its metastable vibrational manifold more efficiently than does helium. C6DH is the major product in the C4H2* + C2D2 reaction, indicating that both C4H2* and C2H2 contribute one hydrogen to the C6H2 product. CS& is observed as a new minor product of the reaction of excited diacetylene with C a 2 , but only at long reaction times. Formation of c 8 H 4 is significantly enhanced by the presence of C2H2. A simple mechanism for the reaction of diacetylene and acetylene with metastable excited diacetylene is formulated. Assuming equal photoionization cross sections for the products, concentration data are used to determine that k(C4H~)/k(C2H2) = 11 f 2 in helium buffer and 7 f 1 in nitrogen buffer, where k(i) is the total rate constant for the reaction of C4H2* with species i.

The chemistry of the triplet metastable state of diacetylene (C 4 H 2 *) with ethene, propene, and propyne in nitrogen and helium buffers is studied in a reaction tube attached to a pulsed nozzle. An ultraviolet photoexcitation laser counterpropagates the molecular expansion through a short reaction tube, exciting the C 4 H 2 1 ∆ u r 1 ∑ g + 2 1 0 6 1 0 and 6 1 0 transitions at 231.5 and 243.1 nm, respectively. Efficient intersystem crossing forms the metstable triplet state from which reaction occurs. The short length of the tube (8 mm) serves to quench the reaction after 10-30 µs so that primary products and not polymer are formed. Upon exiting the reaction tube, the photochemical products are soft ionized with 118 nm vacuum ultraviolet light and mass-analyzed in a linear time-of-flight mass spectrometer. H 4 ) are consistent with poly-yne, enyne, and cumulene products. Percent product yields are determined assuming equal photoionization cross sections for the products. Relative photoionization cross sections at 118 nm for a series of model alkene, alkyne, enyne, diene, and diyne compounds are determined to test the variations in photoionization cross section expected for the products. Relative rate constants for the reactions (scaled to k(C 4 H 2 * + C 4 H 2 ) ) 1.00) with ethene, propene, and propyne are extracted from concentration studies, determining values of 0.24 ( 0.01, 0.32 ( 0.01, and 0.42 ( 0.02 in helium buffer, respectively. Isotopic studies employing deuterated reactants are used to constrain the mechanisms for the reactions. Most of the major products are proposed to follow formation of an unbranched or branched chain adduct which subsequently decomposes by loss of interior atoms to form a stable poly-yne or en-yne product. Two schemes are proposed to account for formation of the isotopically labeled C 5 H 4 and C 5 H 3 products in the C 4 H 2 * + CH 3 C 2 H reaction. Only one of these mechanisms appears to be operative in the C 4 H 2 * + CH 3 CHdCH 2 reaction.