Chemiexcitation transfer to high-lying Rydberg levels of Al

High-lying Rydberg levels of Al were excited in a chemical reaction of aluminum with nitrogen trifluoride. Al atoms were formed in an evaporation furnace,[1],[2] entrained in excess of flowing argon gas, and reacted with NF3. Emission from the flame formed by the reaction was viewed through a suprasil quartz window with a 1 m McPherson scanning monochromator equipped with an EMR 541 E photomultiplier tube. DC photocurrent signals were amplified with a picoammeter and recorded on a strip chart recorder.

An emission spectrum in the 205–230 nm wavelength region from the Al + NF3 reaction, in excess Ar, at 10 torr total pressure is shown in Fig. 1. The doublets in the spectrum belong to the nd2D–3p2p Rydberg series of[3]–[5] Al; emission is observed up to the series limit (marked by a dotted line in the figure). Emission from levels n = 3 up to n = 17 could be resolved. The intensity of this atomic emission increased with argon pressure between 0.5 and 50 torr.

Reactions of Al with F2, SF6, and NF3 at pressures from 0.5 to 50 torr all produced emission from AlF molecules and Al atoms at λ > 227 nm,[6] but only with NF3 was emission from Al at λ < 227 nm observed. A possible mechanism is that high lying levels of Al are populated by energy transfer from excited molecular nitrogen which can be produced in the Al + NF3 mixture by the following sequence of exoergic reactions:

Even if the reactants in reaction (4) are in their ground state, this reaction is exoergic enough to produce N2 (𝐴 Σ3+𝑢, 𝑣≥0) or N2 (𝑋 Σ1+𝑔, 𝑣≥25). An observation that supports the proposed mechanism is the strong green emission from NF b1∑+ at 529 nm and the infrared emission from NF a1Δ at 874 nm which also are seen in the Al + NF3 reaction. In another experiment Al atomic transitions up to the series limit were observed in reaction of Al with microwave discharged N2 at pressures from 1 to 12 torr.

Relative populations of Al states were calculated using observed Al line intensities from our spectra and published transition probabilities.[3] A marked difference between the distribution of Al states in Al+NF3 and that in Al + discharged N2 was found (see Fig. 2). In the latter case approximate Boltzmann distributions of the electronic states with “temperatures” near 3000 K were observed, whereas in Al + NF3 the distribution was non-Boltzmann. A possible explanation for the difference in the electronic state distribution of Al is that the relative amounts of vibrationally excited N2 (𝑋 Σ1+𝑔) and N2 (𝐴 Σ1+𝑢) are different in the two methods of excitation.