Photodissociation of acetylene: Determination of D sup 0 sub 0 (HCC--H) by photofragment imaging

Journal Article · Wed Oct 31 23:00:00 EST 1990 · Journal of Chemical Physics; (USA)

DOI:https://doi.org/10.1063/1.458973· OSTI ID:6286060

Baldwin, D P; Buntine, M A; Chandler, D W ^[1]

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1. Combustion Research Facility, Sandia National Laboratories, Livermore, California 94551 (USA)

Acetylene cooled in a He supersonic expansion is photodissociated by excitation in the 201--216 nm region of the {ital {tilde A}} {sup 1}{ital A}{sub {ital u}} {minus}{ital {tilde X}} {sup 1}{Sigma}{sup +}{sub {ital g}} transition. Subsequent ionization of the H-atom fragments by 2+1 (243 nm) REMPI, and mass-selected ion imaging allows analysis of the velocity distribution of H-atoms from the HCCH {sup {ital h}{nu}}{sub {r arrow}} C{sub 2}H+{ital H} process. Measurement of the maximum velocity for H atoms from this channel produced by photodissociation of acetylene through the {ital {tilde A}} {sup 1}{ital A}{sub {ital u}} {minus}{ital {tilde X}} {sup 1}{Sigma}{sup +}{sub {ital g}} {ital V}{sup 7}{sub 0}{ital K}{sup 1}{sub 0}, 1{sup 1}{sub 0}{ital V}{sup 4}{sub 0}{ital K}{sup 1}{sub 0}, 2{sup 1}{sub 0}{ital V}{sup 5}{sub 0}{ital K}{sup 1}{sub 0} and {ital V}{sup 5}{sub 0}{ital K}{sup 1}{sub 0} vibronic transitions gives a value for {ital D}{sup 0}{sub 0} (HCC--H) of 131{plus minus}1 kcal/mol. Other channels producing hydrogen atoms (including HC{sub 2} {sup {ital h}{nu}}{sub {r arrow}} {ital C}{sub 2}+{ital H} and HCCH{sup {ital h}{nu}}{sub {r arrow}} HCCH{sup +} {sup {ital h}{nu}}{sub {r arrow}} C{sub 2}H{sup +}+{ital H}) are detected at all photon fluxes used. These multiphoton channels produce hydrogen atoms with higher translational energy and therefore obscure measurement of the maximum velocity of H atoms produced by single-photon dissociation of acetylene. Reduction of photon flux by more than two orders of magnitude to {similar to}5{times}10{sup 6} J/cm{sup 2} gives a background, multiphoton, H-atom intensity of {le}7% of the peak primary dissociation intensity. Because this multiphoton background limits the detectability of fast H atoms from single-photon dissociation of acetylene, the dissociation energy reported here is an upper limit.

• OSTI ID: 6286060
• Journal Information: Journal of Chemical Physics; (USA), Journal Name: Journal of Chemical Physics; (USA) Vol. 93:9; ISSN JCPSA; ISSN 0021-9606
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400500* -- Photochemistry
ACETYLENE
ALKYNES
CHEMICAL REACTION YIELD
CHEMICAL REACTIONS
DECOMPOSITION
DISSOCIATION
DISSOCIATION ENERGY
ELECTROMAGNETIC RADIATION
ELEMENTS
ENERGY
ENERGY-LEVEL TRANSITIONS
EXCITATION
FAR ULTRAVIOLET RADIATION
HYDROCARBONS
HYDROGEN
MULTI-PHOTON PROCESSES
NONMETALS
ORGANIC COMPOUNDS
PHOTOCHEMICAL REACTIONS
PHOTOLYSIS
RADIATIONS
ULTRAVIOLET RADIATION
VELOCITY
YIELDS