Allen W. K. Leung

POTENTIAL CURVES FOR SEVERAL ELECTRONIC STATES OF THE MGHE, MG+HE, AND MG+2HE VAN DER WAALS COMPLEXES

We have estimated the potential curves of the Mg(3s3pπ)⋅He[3Π], Mg(3pπ3pπ)⋅He[3Σ−], Mg+(3s)⋅He[2Σ+], Mg+(3pπ)⋅He[2Π], and Mg+2(2p6)⋅He[1Σ+] van der Waals states by means of ab initio calculations. Similar to the analogous doubly excited states of MgNe, MgAr, and MgKr, the Mg(3pπ3pπ)⋅He[3Σ−] state is found to be unusually strongly bound, De=2386 cm−1, a bond strength which is an astounding 165 times that of the singly excited Mg(3s3pπ)⋅He[3Π] state and 35 times that of the Mg+(3s)⋅He ion. The strong bonding is attributed primarily to the lack of a Mg(3s) electron, so that all the attractive forces can extend to smaller internuclear distances because there is no Mg(3sσ)/He(1sσ) repulsion. In fact, the Mg(3pπ3pπ)⋅He[3Σ−], Mg+(3pπ)⋅He[2Π], and Mg+2(2p6)⋅He[1Σ+] states have quite similar bond energies and bond lengths, indicating that for RG=He, the primary attractive force in all these states is the ion/induced-dipole interaction of the “Mg+2/He” core. This is consistent with the fact that the bond energy of ...

An ab initio study of the ground states and some excited states of BeRG, Be+RG, and Be+2RG van der Waals complexes (RG=He, Ne)

By ab initio methods, we have characterized the potential curves of the unusual, doubly excited valence van der Waals states, Be(2pπ+12pπ−1 3PJ)⋅RG[3Σ−], where RG=He, Ne. Similar to the Mg(3pπ+13pπ−1 3PJ)⋅RG[3Σ−] states (RG=He, Ne, Ar, Kr) which were characterized experimentally and theoretically earlier, these Be(2pπ2pπ 3PJ)⋅RG[3Σ−] states are much more strongly bound than their singly excited Be(2s2pπ 3PJ)⋅RG[3Π] analogs, and even much more strongly bound than the analogous Be+(2s 2S)⋅RG[2Σ+] ground-state ions. This is attributed to the lack of a large Be(2s) electron cloud with density along the internuclear axis, so that quadrupole/induced-dipole and dispersion attraction forces can proceed to much smaller internuclear distances before repulsion sets in. The BeHe[3Σ−] state is also almost five times more bound than the BeNe[3Σ−] state, despite the fact that the polarizability of the He atom is only one-half that of the Ne atom. This is again attributed to minimization of repulsive forces, since strong...

Resonant two-color photoionization threshold measurements of the Zn+(4s)⋅Ar bond strength: Model-potential analysis of M+(ns)⋅Ar interactions

The bond energy of the Zn(4s)+⋅Ar ground-state ion has been determined to be De=2085±100 cm−1 by measuring the threshold for two-color resonant photoionization of a ZnAr metastable state. This bond energy is larger (and the estimated bond length is smaller) than the values for other similar M(ns)+⋅Ar states, where M=Mg, Ca, V, Co, Ba, Cd, and Hg, consistent with our earlier postulate that the Zn(4s)+ “core” is smaller than the other M(ns)+ cores in the analogous electronically excited MAr van der Waals complexes for M=Mg, Ca, Zn, Cd, and Hg. We analyze and compare the M+(ns)⋅Ar potential curves using a model potential consisting of calculated 1/R4 (dipole induction) and 1/R6 (quadrupole induction plus dispersion) attractive terms, and an empirically derived Ae−bR repulsion term. The repulsion term for Zn+Ar is consistent with our postulate, in that at smaller R it is the least repulsive of the M+(ns)⋅Ar repulsive potentials when M=Zn, Hg, V, Co, Cd, Mg, Ca, and Ba.

STRONG BONDING IN A DOUBLY EXCITED VALENCE STATE OF A VAN DER WAALS MOLECULE

Abstract The results of ab initio calculations, using extended Gaussian basis sets and including correlation energy through a variety of means, provide strong support for the Mg (3 p π,3 p π 3 P J ) · Ar ( 3 Σ − ) electronic-state assignment made by Massick and Breckenridge to spectral features in the resonant two-photon photoionization spectrum of vibrationally cold Mg (3 s ,3 p π 3 P J ) · Ar ( 3 Π) van der Waals complexes. The doubly excited Mg (3 p π,3 p π 3 P J ) · Ar ( 3 Σ − ) state is found to be much more strongly bound than the singly excited Mg (3 s ,3 p π 3 P J ) · Ar ( 3 Π) state, which, in turn, is much more bound than the Mg (3 s 2 1 S O ) · Ar ( 3 Σ + ) ground state.

The effects of dispersive Cn/Rn-attraction on M+/Rg bonding (M+=atomic metal ion, Rg=rare gas atom)

It has been shown, using a “model-potential” analysis, that −Cn/Rn dispersive terms can be an important part of the physical bonding in M+/Rg complexes (M+=atomic metal ion, Rg=rare gas atom) for M+ ions with large, polarizable outer-shell electron clouds. The model potential equation consists of all attractive terms (accurately calculated or estimated) out to 1/R8, as well as an Ae−bR repulsive term. From known De, Re, and ωe values, and the first and second derivatives of the model potential, the repulsive constants A and b as well as the effective charge Z of M+ in a particular M+⋅Rg electronic state, can be determined. For the typical M+⋅Rg states considered here, Z=1.02±0.07, indicating that no extra “chemical” effects are necessary to explain M+/Rg bonding. Furthermore, the trends in the derived Ae−bR repulsive curves make good qualitative sense. A term-by-term analysis for M+⋅Rg states where the M+ ion is small and unpolarizable [such as Na+(2p6)⋅Rg] shows that −Cn/Rn terms contribute only a few pe...

Potential curves for the ground states and some excited states of MgNe, Mg+Ne, and Mg+2Ne van der Waals complexes

We have estimated the potential curves of the Mg(3s2)⋅Ne(1Σ+), Mg(3s3p)⋅Ne(3Π,3Σ+), Mg(3p2)⋅Ne(3Σ−), Mg+(3s)⋅Ne(2Σ+), Mg+(3p)⋅Ne(2Π), and Mg+2(2p6)⋅Ne(1Σ+) van der Waals states by means of ab initio calculations. Similar to the analogous doubly-excited states of MgAr and MgKr, the Mg(3pπ3pπ)⋅Ne(3Σ−) state is found to be unusually strongly bound, De=548 cm−1, a bond strength which is more than 20 times that of the singly-excited Mg(3s3pπ)⋅Ne(3Π) state and even more than three times that of the Mg+(3s)⋅Ne ion. The strong bonding is attributed primarily to the lack of a Mg(3s) electron, so that all the attractive van der Waals forces can extend to smaller internuclear distances because there is no Mg(3sσ)/Ne(2pσ) exchange repulsion.

Spectroscopic characterization of the metastable 3pπ 3Π0+,0− valence states and the 4s3Σ+ Rydberg states of the MgKr and MgXe van der Waals molecules

The first metastable valence excited states and the first Rydberg states of the MgKr and MgXe molecules have been characterized by resonance two-photon photoionization (R2PI) spectroscopy. The Mg(3s3p 3PJ)⋅RG(3Π0+,0−) metastable states, produced by expanding the products of a laser-ablated magnesium rod in Kr/Ar or Xe/Ar gas mixtures into a supersonic expansion, were excited by a dye laser pulse to several vibrational levels of the Mg(3s4s 3S1)⋅RG(3Σ+) Rydberg states, with detection by ionization with a second dye laser pulse. Spectroscopic constants, bond energies, and bond lengths are reported for both states of MgKr and MgXe. The 3Σ+ Rydberg states are much more strongly bound than the lower 3Π0− valence states, and in fact are essentially as strongly bound as the ground states of the analogous MgRG+ ions, characterized previously in the same apparatus. This clearly indicates that the RG atoms can readily penetrate the diffuse Mg(4s) Rydberg electron cloud. The interesting and unusual spin–orbit and “s...

Spectroscopic characterization of the unusually strongly bound, doubly excited van der Waals state, Mg(3pπ3pπ 3PJ)⋅Kr[3Σ−]

The unusual doubly excited van der Waal’s state, Mg(3pπ3pπ 3PJ)⋅Kr[3Σ−], has been characterized using a laser-vaporization, supersonic-jet source and R2PI (Resonance Two-Photon Ionization) spectroscopy. This state is very strongly bound (De=3966 cm−1) and has a short bond length (Re=2.45 A) compared to its singly excited analogue, Mg(3s3pπ 3PJ)⋅Kr[3Π0−], for which De=267 cm−1 and Re=3.48 A. In fact, this state is even more than twice as strongly bound as the ground-state Mg(3s)+⋅Kr ion, where De=1949 cm−1 and Re≈2.8 A. Possible reasons for the strong van der Waal’s bonding are discussed, and it is concluded that the lack of σ-σ repulsion because there is no Mg(3sσ) valence electron must be a major factor; the similar ionic van der Waal’s state Mg+(3pπ)⋅Kr[2Π], which would be obtained by removing one of the Mg(3pπ) electrons, is even more strongly bound, with De≈7200 cm−1 [J. S. Pilgrim, C. S. Yeh, K. R. Berry, and M. A. Duncan, J. Chem. Phys. 100, 7945 (1994)].

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.

M+/Rg bonding: The effects of M+ permanent quadrupole moments (M+= atomic metal ion; Rg=rare gas atom)

It has been shown, using a model-potential analysis, that the large permanent quadrupole moment of the excited Mg+(3p) ion can play a significant role in the strong physical M+/Rg bonding observed for Mg+(3pπ)⋅Rg[2Π] ionic states. The four permanent quadrupole terms included in the model potential (two proportional to 1/R6, two to 1/R8) contribute substantially to Mg+(3pπ)/Rg attraction near the bond distances Re. In fact, our analysis indicates that the leading charge/induced-dipole 1/R4 attractive term contributes only ∼25–30 % to the physical bonding in the Mg+(3pπ)⋅Ar excited state, in stark contrast to the conventional wisdom that this term is usually dominant in M+/Rg bonding. Empirically derived Ae−bR repulsive terms also show that electron/electron repulsion for a given Mg+(3pπ)⋅Rg excited state is less than for the analogous Mg+(3sσ)⋅Rg ground state, consistent with the fact that the Rg atoms approach the excited 3pπ orbital of Mg+ along its nodal axis. For the Mg+(3pσ)⋅Rg[2Σ+] excited states, ho...