H. Umemoto

Nascent internal energy distributions of MgH(MgD) produced in the reaction of Mg(3s3p1P1) with H2(D2)

Nascent rotational quantum‐state distributions of MgH(v=0,1) and MgD(v=0) have been determined for the reactions Mg(1P1)+H2→MgH+H, Mg(1P1)+D2→MgD+D. The distributions are bimodal, with the major components (∼90%) peaking at very high rotational quantum numbers and the minor components at approximately N=10. The MgH(v=1)/MgH(v=0) ratio is 0.7±0.2, and there is decreasing population in the higher vibrational levels. The ‘‘high‐N’’ distribution is discussed in terms of energy release from bent MgH2 configurations resulting from preferential ‘‘side‐on,’’ insertive attack of H2 by Mg(1P1). This is shown to be consistent with ab initio calculations of the relevant MgH2 potential surfaces. The deconvoluted high‐N distribution for MgD(v=0) is closer to phase‐space‐theory predictions than is that for MgH(v=0), and it is suggested that HMgH and DMgD intermediates are formed with lifetimes nearly long enough for internal randomization of vibrational energy to occur. The minor ‘‘low‐N’’ component could well be due to...

Laser studies of the collisional quenching of electronically excited Mg(3s3p 1P1) atoms

Quenching of the Mg(3s3p 1P1) state by several simple gases has been studied using pulsed laser techniques. Absolute quenching cross sections were determined by pulsed laser excitation of Mg(1P1) and time‐resolved measurements of 2852 A fluorescence. Qualitative identification of major exit channels was accomplished using laser‐induced fluorescence at short time delays. Except for the inert gases and the perfluoroalkanes, the cross sections were approximately gas kinetic and correlated well with the C6 long‐range forces coefficient to the power 0.48±0.05. In contrast to the analogous Cd(5s5p 1P1) level, production of Mg(3PJ) by collision‐induced intersystem crossing was negligible for all quenching molecules studied. Production of MgH was observed for H2, CH4, C3H6, C3H8, and n‐C4H10, but not for C2H2 or C2H4.

Nascent internal energy distributions of MgH produced in the reaction of Mg(3s3p 1P1) with a variety of polyatomic molecules

Nascent rotational quantum state distributions of MgH(v=0,1) produced in the reactions of excited Mg(3s3p 1P1) with a variety of polyatomic molecules have been determined using the laser pump‐and‐probe technique. The distributions are generally bimodal, with one component peaked at low rotational energy and another component at high values of the rotational quantum number N. Except for (CH3)2O, and (C2H5)2O, which produced very little rotationally excited MgH, all compounds with C–H, N–H, and or O–H bonds showed microscopic branching ratios for the ‘‘high‐N’’ component of 0.5±0.1. For the reactants SiH4, GeH4, and PH3, the high‐N branching ratio was ≥0.9. The results are interpreted in terms of competition between side‐on, insertive, and end‐on, abstractive attack of the reagent bonds. Little vibrational energy was found in MgH product for any molecule, consistent with extremely ‘‘late’’ energy release for these heavy–light–heavy kinematic conditions. The results are compared to those from similar studies...

Reaction of Mg(3s3p1P1) with a variety of alkyl C–H bonds: Identical initial MgH(X 2Σ+, v=0) rotational quantum state distributions

The initial distributions of rotational quantum states of MgH(X 2Σ+, v=0) produced in the reaction of electronically excited Mg(3s3p1P1) with a variety of hydrocarbons containing alkyl C–H bonds have been found to be identical within experimental error. The distribution is peaked at fairly low values of rotational angular momentum (N≂10). This is in stark contrast to the reaction of Mg(1P1) with H2, where very high rotational excitation is observed and has been ascribed to side‐on attack of the H–H bond. It is proposed that Mg(1P1) attacks alkyl C–H bonds directly in a relatively narrow cone of angles around the C–H bond axis and that the strength or location of a particular C–H bond has little effect on the dynamical process. Possible reasons for the differences in H2 and hydrocarbon as reactants are discussed, and the hydrocarbon results are compared to those reported by Luntz and co‐workers on the reactions of O(3P) and O(1D2) with alkyl C–H bonds.

Bimodal rotational quantum state distribution of MgH(X 2Σ+, v = 0) in the reaction of Mg(3s3p 1P1) with H2: Evidence for microscopic branching

A ’’pump‐and‐probe’’ technique is employed to determine the initial rotational state distribution of the P1 and P2 branches of MgH. (AIP)

Measurement of unrelaxed-rotational distributions using the laser pump-and-probe technique

Collisional relaxation of the nascent distribution of rotational quantum states of MgH (X 2Σ+, ν = 0) produced in the reactions of H2 and SiH4 with Mg(3s3p 1P1) has been studied by lengthening the delay time in the laser “pump-and-probe” technique. Empirical “scaling lacy” reproduce the relaxation process quite well, and it is shown that the distributions determined at the shortest experimentally accessible delay times are essentially unaffected by secondary rotational relaxation.

REACTIONS OF ZN(4S4P 3P1) AND CD(5S5P 3P1) WITH SIH4

The first nsnp 3P1 excited states of Cd and Zn atoms are shown to readily activate Si–H bonds in SiH4. The nascent quantum state distributions of the CdH(v;N) and ZnH(v;N) products in the reactions of Cd(3P1) and Zn(3P1) with SiH4 have been determined using the laser pump–probe technique. The results are discussed within the context of our current knowledge about the chemical interactions of valence M(nsnp 3P) excited states with Si–H, H–H, and C–H bonds, where M=Mg, Zn, Cd, and Hg. It is proposed that the high reactivity of M(nsnp 3P1) states with H–H and Si–H bonds compared to C–H bonds is simply due to the lack of steric hindrance in the localized, side‐on, M(npπ)–XH(σ*) donor–acceptor molecular orbital interactions, since the Si–H bond‐length in SiH4 is ∼1.5 A compared to C–H bond lengths of ∼1.1 A.

The effective delay time in pump‐and‐probe measurements

The delay time for secondary collisions is considered for systems in which a reactant is created by one short laser pulse, and a product detected with a second short laser pulse at some later time. Sample calculations for the Mg(1P1)+H2→ MgH+H reaction show that the average (or ‘‘effective’’) delay time for secondary collisions of MgH product is different from and not necessarily linearly related to the delay time between the two laser pulses.

Absolute cross sections for the quenching of excited Ca(4s4p 1P1) by several molecules

Abstract Cross sections for the quenching of excited Ca(4s4p 1 P 1 ) by several gases, determined directly by pulsed laser technique, are compared with those of the analogous 1 P 1 states of other group IIa and IIb metal atoms.