A.P. Baronavski

ArF laser photodissociation of NH3 at 193 nm: internal energy distributions in NH2 X̃2B1 and Ã2A1, and two-photon generatin of NH A 3Π and b 1Σ+

Abstract Ammonia is photolyzed with an ArF excimer laser at 193 nm. NH 2 is formed in the A 2 A 1 state, detected through the identification of A 2 A 1 → X 2 B 1 emission bands between 620 and 1100 nm, and also in the X 2 B 1 state, identified by dye laser induced fluorescence on the A ← X transition. The A-state yield is about 2.5% relative to the X-state. The A 2 A 1 state has a propensity for being formed in high K a rotational levels v 2 vibrational levels of the A 2 A 1 state appear to be populated in a monotonically decreasing distribution, with an average vibrational energy content of 1000 cm −1 . Rotational populations are deduced for the 050 Σ vibrational level of the A 2 A 1 state. The colder-than-room-temperature rotational distribution ( T TDI = 210 ± 40 K) is explained by angular momentum conservation arguments. The vibrational and rotational features observed in the dye laser induced fluorescence excitation spectrum of NH 2 A←X could not be assigned at near collision-free pressures. However, under thermalized conditions, the 090 ← 000 Σ, 090 ← 000 Δ, 0 100 - 000 π and 0 100 ← 000 φ bands are clearly identified. Arguments are presented which show that the nascent X 2 B 1 photofragment is created either with the v 2 vibrational population, or the K a rotational population inverted. NH A 3 Π → X 3 Σ − and b 1 Σ + → X 3 Σ − emission are also detected at 336 and 471 nm, respectively. Formation of NH A 3 Π and b 1 Σ + si attributed to two-photon resonance processes

Application of saturation spectroscopy to the measurement of C 2 , 3 Π u concentrations in oxy-acetylene flames

The technique of saturation spectroscopy is applied to measure the concentration of C(2) in the (3)II(u) state in oxyacetylene flames. The two level model is further developed and extended for use in intensity regions slightly lower than that required to saturate the resonances completely. As a result, it has proved feasible to measure both the C(2) number density in the (3)II(u) state of ~10(16)/cm(3) and an excited electronic state lifetime of ~10(-12) sec, both of which depend only on the Einstein A coefficient. The experimental setup is described in detail, and possible extensions of the technique to other atomic and molecular systems are discussed.

Electronic, vibrational and rotational energy partitioning of CN radicals from the laser photolysis of ICN at 266 nm

Abstract A photofragment spectrometer has been designed to directly probe by laser induced fluorescence the internal energy distributions of fragments resulting from unimolecular decomposition. Results for photodissociation of ICN at 266 nm show that CN radicals are formed primarily in the 2 Σ + ground electronic state population distributions: N ν″=1 / N ν″=0 = 0.012, N ν″=2 / N ν″=0 = 6 × 10 −4 . N ν″=3 / N ν″=0 −4 . The rotational energy distribution is estimated to correspond to a rotational temperature of > 3000 K. Less than 0.1% of the CN radicals are formed initially in the A 2 Π electronically excited state.

FTMS studies of mass-selected, large cluster ions produced by direct laser vaporization

Abstract A method is described for the production of large cluster ions by direct laser vaporization in a low-pressure FTMS. Production of high-mass carbon cluster ions (C n + ; 40 n x Sb y + ) cluster ions containing up to five metal atoms are reported. The observed distributions are compared with those obtained previously by both direct laser vaporization and molecular beam sources. Details of the mechanism for formation of these larger cluster ions by direct laser vaporization are discussed. The mass selectivity and long ion residence times obtainable in the FTMS may now be utilized in the study of these cluster ions. Results are presented from a limited study of the ion/molecule reactions and collision induced dissociation of the high-mass carbon cluster ions.

Production of large carbon cluster ions by laser vaporization

The production of high-mass cluster ions of carbon is reported. Using laser vaporization of graphite and time-of-flight mass spectrometric detection, carbon cluster ions, C+n, with n as large as 110, are observed. The cluster ion distribution is similar to that observed for neutral carbon clusters entrained in a supersonic molecular beam and photoionized. Comparisons between the observed cluster ions and the neutral clusters that are generated directly from laser vaporization are made. Results of these experiments are compared with the cluster distributions measured using other methods.

Laser induced photodissociation of Hn3 at 266 nm. I. Primary products, photofragment energy distributions and reactions of intermediates

Abstract A detailed analysis of the primary photodissociation products resulting from the 266 nm laser photolysis of HN 3 is reported. The major primary fragments are N 2 ( 1 Σ g + ) and NH( 1 Δ). The NH( 1 Δ) fragment is formed ≳ 99.8% in the ν = 0 level with ≈ 900 cm −1 of rotational energy and ⩽ 5000 cm −1 of translational energy for the axially scattered fragments. A new chemiluminescent reaction is reported: NH( 1 Δ) + HN 3 ( 1 A′) → NH 2 ( 2 A 1 ) + N 3 ( 2 Π g ), which appears to be a major reaction channel of the primary NH( 1 Δ) fragment. A kinetic analysis of this reaction and several other NH( 1 Δ) reactions are the subject of the following associated paper. A correlation study of the NH( 1 Δ) and N 2 ( 1 Σ g + ) products with the dissociating states of HN 3 is made which requires a reassignment of the lower-lying HN 3 transitions.

Wavelength dependence of the I(52P12,32) branching ratio from ICN Ã state photolysis

Quantum yields for the production of I(52P12) atoms from the photolysis of ICN in the A state continuum are presented as a function of wavelength. Evidence is presented for the existence of at least three electronic states giving rise to the absorption spectra. Primary photophysical processes involved in the dissociation of ICN are discussed.

Excited state dynamics and bimolecular quenching processes for NH2(Ã2A1)

Abstract Radiative lifetimes and quenching rate constants have been measured for the A 2 A 1 state of NH 2 , formed through ArF excimer laser photolysis of NH 3 at 193 nm. The NH 2 (A 2 A 1 → X 2 B 1 ) emission is discrete, allowing single vibrational and rotational levels to be isolated. An average radiative lifetime of 31 ± 4 μs and an NH 3 quenching rate constant of (6.1 ± 0.2) × 10 −10 cm 3 molecule −1 s −1 is found to be independent of the K ″ a rotational level and ν′ 2 vibrational level for K ″ a = 1, 4, 5 and 6 and ν′ 2 = 6, 7 and 8. The 050 Σ level ( K ′ a = 0) lifetime is slightly longer (46 ± 6 μs) and its quenching rate constant somewhat lower [(5.0 ± 0.2) × 10 −10 cm 3 molecule −1 s −1 ]. These lifetimes are interpreted as radiative lifetimes of levels of the A 2 A 1 state, unperturbed by Renner—Teller coupling with the ground state. The quenching by NH 3 is shown to be predominantely electronic quenching of the A 2 A 1 state, with vibrational and rotational relaxation within the excited state being at least ten times slower. A second, previously unobserved, long-lived component is found in the decay of broad-band emission, comprising about 15% of the fluorescence intensity between 620 and 890 nm. Its lifetime is ⩾ 100 μs, coupling between some levels of the A 2 A 1 and X 2 B 1 states.

Photofragment energy distributions and reaction rates of NH from photodissociation of HN3 at 266 nm

Abstract Detailed analyses of the electronic, vibrational, and rotational distributions of NH formed by photolysis of HN3 at 266 nm are given. NH is formed primarily in the a (1Δ, ν = 0) state with ≈900 cm−1 state with ≈900 cm−1 rotational energy and ⪅ 5000 cm− of translational energy. In addition, a new chemiluminescent reaction is reported: NH(1Δ) + NH3 → NH2(2A1) + N3. The rate constant for this reaction is in agreement with an earlier measurement of the disappearance of NH(1Δ) by an absorption technique.

Laser induced photodissociation of Hn3 at 266 nm. II. Reactions of NH(1Δ) with HN3 HCl and hydrocarbon species

Abstract In the preceding paper it was shown that the 266 nm photodissociation of HN3 gives rise to NH fragments exclusively in the vibrationless a(1Δ) state with about 900 cm−1 of rotational energy. These fragments collisionally react with HN3 to produce NH2(2A1) in a chemiluminescent reaction. The time resolved chemiluminescence emission is used to determine the reaction rate for NH(1Δ) + HN3 → NH2(2A1) + N3(2Πg). The reaction rates of NH(1Δ) with several other species, HCl, CH4, C2H4, C3H6 and C6H12 are reported. Possible mechanisms for these reactions are considered. Condensed phase experiments are reported describing the addition reaction of NH(1Δ) with cyclohexane.