High-resolution observations of the infrared spectrum of neutral neon

Journal of Research of the National Institute of Standards and Technology, May-June, 2004 by Craig J. Sansonetti, Marion M. Blackwell, E.B. Saloman

We have observed the spectrum of neutral neon (Ne I) emitted by a microwave-excited electrodeless discharge lamp with the National Institute of Standards and Technology 2 m Fourier transform spectrometer. The spectra cover the regions 6929 [Angstrom] to 11 000 [Angstrom] with a resolution of 0.01 [cm.sup.-1] and 11 000 [Angstrom] to 47 589 [Angstrom] with a resolution of 0.007 [cm.sup.-1]. We present a line list that includes more than 650 classified lines and provides an accurate and comprehensive description of the infrared spectrum. The response of the Fourier transform spectrometer was determined by using a radiometrically calibrated tungsten strip lamp, providing relative intensities that for moderate to strong lines are accurate to approximately 10% over the entire range of the observations. The identities of many lines that were previously multiply classified are unambiguously resolved.

Key words: atomic spectroscopy; Fourier transform spectroscopy; infrared; neon; wavelengths.

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1. Introduction

Neon discharges are widely used in scientific, technical, and commercial applications. Despite the fact that Ne is frequently used as a buffer gas in sources for laboratory spectroscopy, no comprehensive description of its spectrum in the extraphotographic infrared region has appeared in the literature. Photographic spectra recorded with large grating spectrographs in the near infrared region have reasonably high resolution but extend to only about 12 000 [Angstrom] [1]. At longer wavelengths, spectra obtained with infrared scanning instruments have resolution and accuracy that are very low by current standards [2,3].

The most complete line list for Ne at wavelengths longer than 11 000 [Angstrom] was given by Humphreys [4] based on spectra recorded with a scanning 1 m grating spectrometer. The relative intensities in this list are experimental values from Humphreys's grating observations, but the wavelengths are calculated from the Ne energy levels of Kaufman and Minnhagen [5]. Because of the low resolution and accuracy of the experimental spectra, many lines are multiply classified. In some cases as many as five classfications and calculated wavelengths are associated with a single feature in the observed spectrum. For such a case Ref. [4] provides no information about the relative intensities of these possible transitions in a high resolution spectrum, nor does it provide any basis for estimating the effective wavelength of the unresolved transitions at low resolution.

Measurements for 118 infrared lines of neon were reported by Chang et al. [6] based on hollow cathode spectra from the archives of the Fourier transform spectrometer of the National Solar Observatory at Kitt Peak National Observatory. These are the most accurate published measurements for neon in the infrared. Unfortunately, the work of Chang et al. has little value as a description of the spectrum since it gives no intensities and includes only selected lines.

In order to provide a comprehensive high-resolution description of the infrared spectrum of Ne, we made new observations with the National Institute of Standards and Technology (NIST) 2 m Fourier transform spectrometer (FTS). This is one component of a broader program of observations and compilations for the noble gases currently in progress at NIST. A more detailed description of this program and of our experimental observations for the infrared spectra of the noble gases has been given in Ref. [7]. We have also produced a complete compilation of transitions and energy levels for Ne I [8], which includes many data from the present work.

2. Experiment

The spectrum was excited in a commercial sealed electrodeless discharge lamp filled for this work with Ne at a pressure of 200 Pa (1.5 Torr). The lamp design, illustrated in Fig. 1, is derived from that used by Wilkinson and Tanaka [9]. The lamp is equipped with a cemented magnesium fluoride window that permits viewing along the axis of the discharge. The double wall in the area of the window protects the epoxy seal from the discharge as demonstrated by Bass [10]. The lamp contains a barium getter in a side arm to trap impurities released from the walls during operation. Initial attempts to power the lamp using an Evenson cavity [11] were unsuccessful because we could not maintain stable tuning of the cavity over the several hours required for data acquisition. This resulted in large discontinuous variations in light output that were unacceptable for the FTS. We therefore turned to a cuptype antenna directed toward the side of the lamp near its midpoint to couple power to the discharge. This produced very stable excitation.

[FIGURE 1 OMITTED]

Light from the electrodeless lamp was directed to the entrance aperture of the FTS through a path purged with dry air to avoid infrared absorption by atmospheric water vapor. The purged path incorporates a remotely-actuated rotating mirror and a concave mirror as shown in Fig. 2, permitting light from either the Ne lamp or a radiometric standard source to be imaged to the entrance aperture of the FTS. The radiometric standard source, a tungsten strip lamp with sapphire window, was used for calibration of the instrumental response.