Rydberg and autoionizing state spectroscopy of laser-ablated carbon

Small carbon species such as atomic carbon, C2, and C3 are important in combustion, the atmospheres of some stars, the interstellar medium, and as intermediates in the production of larger carbon clusters such as fullerenes. The spectroscopy of these species has a long history, but has always been hampered by the difficulty of producing them in the gas phase with sufficient density. The use of laser ablation of graphite provides a simple method for circumventing this problem.


Remarkably, the high Rydberg states of atomic carbon have not previously been studied through the use of laser spectroscopy; in addition, the states involving an np Rydberg electron (with orbital angular momentum quantum number of 1) which are inaccessible from the ground state by the absorption of one photon have not even been considered in any detail theoretically.


We have studied the high principal quantum number 2pnp Rydberg states of atomic carbon converging to the two spin-orbit states of the ground state of the ion. Carbon atoms produced in a laser ablation process were excited through a laser-driven two-color resonantly enhanced scheme using tunable pulsed VUV light to excite the resonance transitions, followed by further excitation to the Rydberg states. Results of this work include a re-determination of the ionization energy of the atom and measurements of series quantum defects and interseries coupling strengths.

A beam of atomic carbon was produced by laser ablation of a rotating graphite rod using 20 mJ, 5 ns pulses of light at 1.064 um, focused loosely. The ablation plume expanded into a vacuum region which contained the field plates for a time-of-flight ion mass spectrometer. This vacuum region was pumped through a 3 mm diameter hole in the plate leading to the mass spectrometer flight tube, which was itself evacuated by a turbomolecular pump.
Figure 1.
High np states of the atomic carbon vapor were excited through a 1+1' process utilizing resonance with the upper states of the resonance line transition, 2p2 3P0,1,2 ->2p3s 3P0,1,2, as depicted in figure 2. The required
Figure 2.
VUV radiation at ~166 nm was produced by two-photon resonant difference-frequency generation in xenon gas, utilizing the output from two homebuilt tunable dye lasers pumped by a Nd+3:YAG laser. The VUV light had a bandwidth of ~0.2 cm-1, duration of ~3 ns, a 2 mm beam diameter at the carbon beam, and an estimated intensity of ~1011 photons/pulse. Figure 3 shows the observed spectrum of 2p2 3PJ"-2p3s 3PJ' transitions as the VUV wavelength was scanned. After excitation of a particular J' level of the 2p3s configuration, tunable light from a third homebuilt, frequency-doubled tunable dye laser at ~330 nm and ~0.05 cm-1 bandwidth was used to probe the 2p3s-2pnp Rydberg transitions. The Rydberg states were field-ionized using a 170 V/cm pulse, delayed by 100 ns from the excitation laser pulses. The maximum C+ signal was obtained when the excitation lasers were delayed from the ablation laser by 125 us, indicating a translational temperature of ~4000 deg.K for the carbon atoms. The spectra were calibrated using a simultaneously acquired spectrum of molecular iodine to an accuracy of ~0.05 cm-1.

Figure 3.

Figure 4.

Figure 4 shows a spectrum of transitions obtained by exciting from the J'=2 level to the region of the ionization limit. The intense, sharp peaks are due to excitation of the 3D3 Rydberg series converging to the 2P3/2 ion core state. Since there is no other J=3 series possible for the 2pnp configuration, these states do not autoionize, and consequently are not broadened or otherwise perturbed. Therefore, their energies can be fit accurately to a simple Rydberg formula to yield the energy difference between the 2p3s J'=2 state and the 2P3/2 ionization limit. Such a fit yields the result 30490.55 +/- 0.02 cm-1. When combined with an improved measurement of the 2p3s level energies, this result will provide a new determination of the ionization energy of carbon with significantly improved accuracy compared to the best existing value.

Figure 5 shows the J=1 Rydberg spectrum excited from the J'=0 level of the 2p3s state. Theory predicts that two series converging to each of the ionization limits should be seen. This spectrum is a beautiful example of Rydberg state channel interactions, and a collaborative effort to produce a theoretically predicted spectrum to compare with experiment is planned, with Francis Robicheaux at Auburn University.

Figure 5.
The combined techniques of laser ablation and VUV+visible or UV stepwise excitation are very powerful, and will enable studies of a number of other refractory atomic and molecular species.

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