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The Journal of Chemical Physics

Observation of collisional ionization of chromium atoms in high Rydberg states and the N4 scaling relation

Daniel T. Rooney, Joseph Chaiken

Published Jul 1, 1990• 2 citations

Abstract

We have used mass selected multiphoton dissociation/excitation/collisional ionization (MPD/MPE/CI) spectroscopy to study benzene chromium tricarbonyl (BCT) in an effusive molecular beam under collisionless and collisional conditions. Under collisionless conditions, our results are entirely consistent with, but not identical to, our earlier nonmass selected published experiments under bulk gas conditions. The spectroscopic results confirm that we can produce chromium atoms in a variety of well defined Rydberg states (7S3[Ns3d5], N=30–43). Following the state selective preparation of the chromium Rydberg atoms, by MPD of BCT followed by MPE of the ground state chromium atoms, they can be allowed to collide with ground electronic state benzene chromium tricarbonyl molecules producing ground state chromium ions and other products. We observe the scaling of the cross section for this collisional ionization process as a function of the Rydberg electron principal quantum number. The cross sections are a strong function of the Rydberg electron principal quantum number N, even when N is very large. Scaling as N4, the cross sections suggest that dipolar interactions with molecular colliders can be quite efficient at inducing collisional ionization of Rydberg atoms.

DOI: 10.1063/1.459546

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We have used mass selected multiphoton dissociation/excitation/collisional ionization (MPD/MPE/CI) spectroscopy to study benzene chromium tricarbonyl (BCT) in an effusive molecular beam under collisionless and collisional conditions. Under collisionless conditions, our results are entirely consistent with, but not identical to, our earlier nonmass selected published experiments under bulk gas conditions. The spectroscopic results confirm that we can produce chromium atoms in a variety of well defined Rydberg states (7S3[Ns3d5], N=30–43). Following the state selective preparation of the chromium Rydberg atoms, by MPD of BCT followed by MPE of the ground state chromium atoms, they can be allowed to collide with ground electronic state benzene chromium tricarbonyl molecules producing ground state chromium ions and other products. We observe the scaling of the cross section for this collisional ionization process as a function of the Rydberg electron principal quantum number. The cross sections are a strong function of the Rydberg electron principal quantum number N, even when N is very large. Scaling as N4, the cross sections suggest that dipolar interactions with molecular colliders can be quite efficient at inducing collisional ionization of Rydberg atoms.

Thermal collisions between laser excited Xe(nf) Rydberg atoms and a variety of neutral atomic and molecular targets have been investigated. In Xe(nf)-HF collisions, n-changing, 4-changing and collisional ionization were observed. For n = 26 and 27, absolute collisional cross sections for these processes have been determined using time-resolved selective field ionization. In addition, the first evidence of molecular rotational excitation through the transfer of energy of electronic excitation from a Rydberg atom is presented. In Xe(nf) - Xe, Kr, Ar, CO2, CO and N2 collisions the dominant mechanism was observed to be 4-changing: no evidence of n-changing or collisional ionization was apparent. Absolute collisional cross sections have been determined for a range of n between 24 and 37. Comparison of these cross sections with those obtained using a theoretical model in which effects due to the Rydberg core are ignored appears to lead to satisfactory results only for Xe, Kr and CO2, whereas when core effects are included satisfactory agreement between experiment and theory is found in all cases.

The ionization of He atoms, prepared in a well-defined (n=14)

^{1}

P Rydberg state, in collisions with electron-attaching molecules like

{\mathrm{SF}}_{6}

and

{\mathrm{NO}}_{2}

, is studied in a crossed-beam experiment at thermal energies. The ionization cross section is observed to be large for

{\mathrm{SF}}_{6}

, reaching 4.2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}13}$

{\mathrm{cm}}^{2}

, and to be smaller for

{\mathrm{NO}}_{2}

, close to ${10}^{\mathrm{\ensuremath{-}}15}$

{\mathrm{cm}}^{2}

. The dependence of these two cross sections on the relative velocity of the reactants exhibits a characteristic behavior with a saturation at low velocities, a dependence that is completely different from that obtained with polar molecules. The observation of such a dependence for

{\mathrm{NO}}_{2}

indicates that, for this molecule, the reaction product is an ion pair, as for

{\mathrm{SF}}_{6}

, and that ionization is not due to rotational energy transfer even though

{\mathrm{NO}}_{2}

has a dipole moment. This dependence cannot be explained by the ``free-electron model'' usually used in Rydberg-atom--molecule collisions; it clearly shows that the interaction of the molecule with the ion core cannot be neglected.

Highly excited (Rydberg) atoms are produced and can exist for significant periods of time in various plasma environments. It is necessary to know cross sections for the various collision processes involving these atoms if a complete understanding of the physics of these environments is to be attained.\nThe experimental investigation of collisional phenomena between high Rydberg sodium atoms and electrons is described in this thesis. A fraction of the atoms in a ground state sodium beam is optically excited (using lasers) to well-defined, high Rydberg states (principal quantum numbers chosen around 40). Subsequent to their production the Rydberg atoms are allowed to interact with 25 eV electrons for a specified time interval following which the collision products are detected. The electrons are produced in a beam which is crossed at right angles with the sodium beam. Important collision processes are (1) electron impact ionization of the Rydberg atom, and (2) electron-Rydberg atom interactions in which the quantum state of the atom is changed (state changing collisions).\nResults are presented for an observed state changing process which is identified as angular momentum state changing only (no change in the value of the principal quantum number). Cross sections for this process are measured to be 10('-10) to 10('-9) cm('2). Problems encountered during the work which prevented the measurement of electron impact ionization cross sections are discussed. In the course of the work two other topics required study: (1) the process of electric field ionization of Rydberg atoms (used to detect the atoms), and (2) the effects of background (300 K) radiation (from the chamber walls, for example) on Rydberg atoms. Results of these studies are also presented.

We showed that there are two main processes that produce the collisional background in the method of the collinear laser photoionization of fast atoms. The first process is the ionization of fast atoms and the second one is the collisional excitation of fast atoms into Rydberg states.The first experimental results on Rydberg states formation cross sections in atomic collisions were obtained. For the first time it has been experimentally obtained such cross sections scaled as n−3.To reduce the collisional noise the use of fast Rydberg atoms ionization in permanent magnetic field is proposed.

The total collisional cross sections of caesium Rydberg 40 S state is obtained by measuring the corresponding effective decay rates as a function of Rydberg atoms density in a conventional magneto-optical trap using the state-selective pulse field ionization technique. We investigate the collisional properties for different Rydberg atomic densities in detail. The results indicate that the Rydberg atomic interaction induced collisions is the dominant mechanism leading to the large collisional cross sections.