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Register a new userIonization of Rydberg H atoms at band-gap metal surfaces via surface and image states
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Wavepacket propagation calculations are reported for the interaction of a Rydberg hydrogen atom ($n=2-8)$ with Cu(111) and Cu(100) surfaces (represented by a Chulkov potential), in comparison with a Jellium surface. Both copper surfaces have a projected band gap at the surface in the energy range degenerate with some or all of the Rydberg energies. The charge transfer of the Rydberg electron to the surface is found to be enhanced for $n$ values at which there is a near-degeneracy between the Rydberg energy level and an image state or a surface state of the surface. The enhancement is facilitated by the strong overlap of the surface image-state orbital lying outside the surface and the orbital of the incoming Rydberg atom. These calculations point to the possibility of using Rydberg-surface collisions as a probe of surface electronic structure.
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The interface formation between copper phthalocyanine (CuPc) and two representative metal substrates, i.e., Au and Co, was investigated by the combination of ultraviolet photoelectron spectroscopy and inverse photoelectron spectroscopy. The occupied and unoccupied molecular orbitals and thus the transport band gap of CuPc are highly influenced by film thickness, i.e., molecule-substrate distance. Due to the image charge potential given by the metallic substrates the transport band gap of CuPc opens from $(1.4 pm 0.3)$ eV for 1 nm thickness to $(2.2 pm 0.3)$ eV, and saturates at this value above 10 nm CuPc thickness. The interface dipoles with values of 1.2 eV and 1.0 eV for Au and Co substrates, respectively, predominantly depend on the metal substrate work functions. X-ray photoelectron spectroscopy measurements using synchrotron radiation provide detailed information on the interaction between CuPc and the two metal substrates. While charge transfer from the Au or Co substrate to the Cu metal center is present only at sub-monolayer coverages, the authors observe a net charge transfer from the molecule to the Co substrate for films in the nm range. Consequently, the Fermi level is shifted as in the case of a p-type doping of the molecule. This is, however, a competing phenomenon to the energy band shifts due to the image charge potential.
The charge transfer (ionization) of hydrogen Rydberg atoms (principal quantum number $n=25-34$) incident at a Cu(100) surface is investigated. Unlike fully metallic surfaces, where the Rydberg electron energy is degenerate with the conduction band of the metal, the Cu(100) surface has a projected bandgap at these energies, and only discrete image states are available through which charge transfer can take place. Resonant enhancement of charge transfer is observed at hydrogen principal quantum numbers for which the Rydberg energy matches the energy of one of the image states. The integrated surface ionization signals show clear periodicity as the energies of states with increasing $n$ come in and out of resonance with the image states. The velocity dependence of the surface ionization dynamics is also investigated. Decreased velocity of the incident H atom leads to a greater mean distance of ionization and a lower field required to extract the ion. The surface-ionization profiles (signal versus applied field) for `on resonance $n$ values show a changing shape as the velocity is changed, reflecting the restriction of the resonance to a certain range of applied field.
Helium atoms in Rydberg states have been manipulated coherently with microwave radiation pulses near a gold surface and near a superconducting NbTiN surface at a temperature of $3 text{K}$. The experiments were carried out with a skimmed supersonic beam of metastable $(1text{s})^1(2text{s})^1, {}^1text{S}_0$ helium atoms excited with laser radiation to $ntext{p}$ Rydberg levels with principal quantum number $n$ between $30$ and $40$. The separation between the cold surface and the center of the collimated beam is adjustable down to $250 mutext{m}$. Short-lived $ntext{p}$ Rydberg levels were coherently transferred to the long-lived $ntext{s}$ state to avoid radiative decay of the Rydberg atoms between the photoexcitation region and the region above the cold surfaces. Further coherent manipulation of the $ntext{s}$ Rydberg levels with pulsed microwave radiation above the surfaces enabled measurements of stray electric fields and allowed us to study the decoherence of the atomic ensemble. Adsorption of residual gas onto the surfaces and the resulting slow build-up of stray fields was minimized by controlling the temperature of the surface and monitoring the partial pressures of H$_2$O, N$_2$, O$_2$ and CO$_2$ in the experimental chamber during the cool-down. Compensation of the stray electric fields to levels below $100 text{mV}/text{cm}$ was achieved over a region of $6 text{mm}$ along the beam-propagation direction which, for the $1770 text{m}/text{s}$ beam velocity, implies the possibility to preserve the coherence of the atomic sample for several microseconds above the cold surfaces.
We have studied an ionization of alkali-metal Rydberg atoms by blackbody radiation (BBR). The results of the theoretical calculations of ionization rates of Li, Na, K, Rb and Cs Rydberg atoms are presented. Calculations have been performed for nS, nP and nD states which are commonly used in a variety of experiments, at principal quantum numbers n=8-65 and at the three ambient temperatures of 77, 300 and 600 K. A peculiarity of our calculations is that we take into account the contributions of BBR-induced redistribution of population between Rydberg states prior to photoionization and field ionization by extraction electric field pulses. The obtained results show that these phenomena affect both the magnitude of measured ionization rates and shapes of their dependences on n. A Cooper minimum for BBR-induced transitions between bound Rydberg states of Li has been found. The calculated ionization rates are compared with our earlier measurements of BBR-induced ionization rates of Na nS and nD Rydberg states with n=8-20 at 300 K. A good agreement for all states except nS with n>15 is observed. Useful analytical formulas for quick estimation of BBR ionization rates of Rydberg atoms are presented. Application of BBR-induced ionization signal to measurements of collisional ionization rates is demonstrated.
A new scheme is proposed for modeling molecular nonadiabatic dynamics near metal surfaces. The charge-transfer character of such dynamics is exploited to construct an efficient reduced representation for the electronic structure. In this representation, the fewest switches surface hopping (FSSH) approach can be naturally modified to include electronic relaxation (ER). The resulting FSSH-ER method is valid across a wide range of coupling strength as supported by tests applied to the Anderson-Holstein model for electron transfer. Future work will combine this scheme with ab initio electronic structure calculations.
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