M. Mangir

The kinetics of free radicals generated by IR laser photolysis. II. Reactions of C2(X 1Σ+g), C2(a 3Πu), C3(X̄ 1Σ+g) and CN(X 2Σ+) with O2☆

Abstract The 300 K reactions of O 2 with C 2 (X 1 Σ + g ), C 2 (a 3 Π u ), C 3 (X 1 Σ + g ) and CN(X 2 Σ + ), which are generated via IR multiple photon dissociation (MPD), are reported. From the spectrally resolved chemiluminescence produced via the IR MPD of C 2 H 3 CN in the presence of O 2 , CO molecules in the a 3 Σ + , d 3 Δ i , and e 3 Σ − states were identified, as well as CH(A 2 Δ) and CN(B 2 Σ + ) radicals. Observation of time resolved chemiluminescence reveals that the electronically excited CO molecules are formed via the single-step reactions C 2 (X 1 Σ + g , a 3 Π u ) + O 2 → CO(X 1 Σ + + CO(T), where T denotes are electronically excited triplet state of CO. The rate coefficients for the removal of C 2 (X 1 Σ + g ) and C 2 (a 3 Π u ) by O 2 were determined both from laser induced fluorescence of C 2 (X 1 Σ + g ) and C 2 (a 3 Π u ), and from the time resolved chemiluminescence from excited CO molecules, and are both (3.0 ± 0.2)10 −12 cm 3 molec −1 s −1 . The rate coefficient of the reaction of C 3 with O 2 , which was determined using the IR MPD of allene as the source of C 3 molecules, is −14 cm 3 molec −1 s −1 . In addition, we find that rate coefficients for C 3 reactions with N 2 , NO, CH 4 , and C 3 H 6 are all −14 cm 3 molec −1 s −1 . Excited CH molecules are produced in a reaction which proceeds with a rate coefficient of (2.6 ± 0.2)10 −11 cm 3 molec −1 s −1 . Possible reactions which may be the source of these radicals are discussed. The reaction of CN with O 2 produces NCO in vibrationally excited states. Radiative lifetime of the Ā 2 Σ state of NCo and the Ā 1 Π u (000) state of C 3 are reported.

Kinetics of free radicals generated by IR laser photolysis. IV. Intersystem crossings and reactions of C2(X 1Σ+g) and C2(a 3Πu) in the gaseous phase

Rate coefficients (300 K) for the removal of C2(X 1Σ+g) and C2(a 3Πu), hereafter referred to as 1C2 and 3C2 respectively, by H2, NO, and a number of hydrocarbons are reported as well as rate coefficients for intersystem crossing between 1C2 and 3C2 induced by collisions with N2, CO2, CF4, Ar, Kr, and Xe. C2 molecules are produced via ir photolysis of C2H3CN or C2HCl3, and their concentrations are monitored by laser induced fluorescence. We find that collisionally induced intersystem crossing is significant only when it is spin allowed or involves heavy collision partners (e.g., Kr, Xe). 1C2 reacts more rapidly with NO than does 3C2, and excited CN molecules in the A and B states are formed predominantly in reactions of 3C2. 1C2 reactions result (mainly) in ground state CN, as expected from adiabatic state correlations. Radiationless transitions between the X and B states of CN, induced by collisions with Ar, are observed. It is suggested that both 1C2 and 3C2 are removed by hydrocarbons mainly via chemica...

The kinetics of free radicals generated by IR laser photolysis. I. Reactions of C2(a 3Πu) with NO, vinyl cyanide, and ethylene

C2(a 3Πu) molecules are produced by multiple photon dissociation of either vinyl cyanide or ethylene with the focused output from a CO2 TEA laser. Their reaction with NO is reported in this paper. The total quenching rate coefficient of C2(a 3Πu) by NO is determined both from laser induced fluorescence of C2(a 3Πu) and from the time resolved chemiluminescence from CN(B 2Σ+) and CN(A 2Π) which are formed in the reaction, and is (7.5±0.3)10−11 cm3 molecule−1 sec−1. The vibrational energy disposal within the B 2Σ+ and A 2Π states of CN is estimated from the resolved bands of the CN(B 2Σ+→X 2Σ+) and CN(A 2Π→X 2Σ+) emission systems. Vibrational levels of CN(B 2Σ+) up to at least v′=5 but lower than v′=10 are excited, accompanied by high rotational excitation. CN in the A 2Π state is excited to at least the 16th vibrational level. The reaction populates the A 2Π state about 7 times more efficiently than it does the B 2Σ+ state. The reaction mechanism is discussed, and it is concluded that most of the electronic...

The kinetics of free radicals generated by IR laser photolysis. III. Intersystem crossing between C2(X 1Σg+) and C2(a 3Πu) induced by collisions with oxygen

Reactive and energy transfer processes involving collisions of C2(X 1Σg+) and C2(a 3Πu) with molecular oxygen are reported. Intersystem crossing between C2(X 1Σg+) and C2(a 3Πu) is induced very efficiently by collisions with O2. The triplet→singlet rate coefficient is (2.7+0.3−0.6)×10−11 cm3molecule−1 s−1 and the ratio between this and the singlet→triplet rate coefficient is ∼3 at 300 K. The rate coefficient for reaction is much less than that for intersystem crossing, making the measurement of separate singlet and triplet reaction rate coefficients with oxygen impossible. The previously measured reaction rate coefficient (3×10−12 cm3molecule−1 s−1 at 300 K) for both singlet and triplet C2 removal by O2 is reinterpreted as the rate coefficient for removal of equilibrated C2 molecules. Using hydrocarbon scavengers, it is shown that the source of the observed emissions from excited triplet states of CO is mainly reactions of C2(X 1Σg+) with O2.

Collisionless production of electronically excited species via ir laser photolysis

Abstract In the absence of collisions, luminescence is observed upon irradiation of vinyl cyanide molecules with the focused output from a CO 2 TEA laser, namely, two broad, structureless features, at 390 nm and in the IR whose intensity depends linearly on pressure from 10 −4 to 10 −2 torr. At higher pressures, a collision induced emission component appears. The time resolved fluorescence reveals a fast rise, never longer than the CO 2 laser pulse duration, and a much slower decay. The luminescence is believed to originate from a dissociation fragment.

Vibrational relaxation in HgBr(X2∑12+)

Abstract We report a study of the net collisional removal of HgBr(X2∑ 1 2 +, v″ = 22) by O2, N2, H2, He, Ne, Ar, Kr, and Xe. Rate coefficients are large, indicating that deactivation of the lower laser levels in the HgBr(B → X) lasers proceeds efficiently.

Infrared Laser Photolysis Of Polyatomic Molecules: A Powerful Technique For Studying Elementary Processes In The Gas Phase

The infrared photolysis of polyatomic molecules is discussed. Several examples are presented, with products being detected either by chemiluminescence or by laser-induced fluorescence. The technique allows a study both of the dynamics of unimolecular decomposition and of the reaction kinetics and dynamics of the resulting free radicals. Information on the former is deduced from such features as product energy partitioning, product yield as a function of incident laser fluence, and the effect of collisions on the degree and rate of the dissociation. The latter aspect of the technique is powerfully illustrated by the reactions of C2 (X,a) radicals (produced by photolysis of several organic molecules) with such gases as 02 and NO, from which chemiluminescent reaction products have been detected.

Optical detection of photoproducts using a pulsed supersonic molecular beam: Application to intramolecular V ⇄ E coupling in IR laser excited polyatomics

It is well established that electronic emission often accompanies the IR multiple photon excitation of polyatomic species in the presence of a strong electromagnetic field. We have used a pulsed supersonic molecular beam arrangement to study this phenomenon for the case when propenenitrile is irradiated with the focused output from a TEA CO2 laser. Electronically excited species prepared in this way have long spontaneous emission lifetimes and can therefore be detected, via their emission, downstream from the intersection of the laser and molecular beam axes. By measuring time of flight and angular distributions, we can obtain the center‐of‐mass recoil velocity distribution of the emitting species. For the case of propenenitrile, the recoil velocity distribution is peaked at 250 m s−1 with a width of approximately 200 m s−1. Details of the multistep dissociation process are discussed, and we conclude that H2 molecular elimination precedes formation of the emitting species, which is either C2HCN or C2CN.

Intramolecular Processes in Isolated Polyatomic Molecules

This paper deals with certain rather well known unimolecular processes which occur in isolated polyatomic molecules. These processes are (i) chemical reactions, forming fragments with identifiable internal and translational energy distributions, and (ii) the coupling of vibrational and electronic degrees of freedom, a phenomenon which is responsible for so-called radiationless transitions. The work is mainly experimental, and capitalizes on the fact that infrared lasers can be used to excite the vibrations of polyatomics in the absence of collisions. Thus, parent translational and rotational energies are near ambient, while vibrations can be excited rather easily, allowing us to measure elementary unimolecular processes of these species without interference from collisions. Although molecular excitation cannot be adjusted to yield monoenergetic species of excitation, still the “average excitation level” can be controlled, enabling us to determine effects qualitatively. Above dissociation threshold, the balance between the optical pumping rate and the dissociation rate results in a rather narrow range of energies from which dissociation occurs, and by adjusting the laser intensity the mean unimolecular rate can be controlled.