A Doppler-limited rubidium atlas in ascii format, 9500–12 300 cm−1

Journal of Molecular Spectroscopy 264 (2010) 78–81

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A Doppler-limited rubidium atlas in ascii format, 9500–12 300 cm1 Amanda J. Ross ⇑, Victor Bertrand, Heather Harker, Patrick Crozet Université de Lyon, F-69622, Lyon, Université Lyon 1 & CNRS, UMR 5579 LASIM, 43 Bd du 11 novembre 1918, Villeurbanne, France

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Article history: Received 3 August 2010 In revised form 15 September 2010 Available online 24 September 2010 Keywords: Rubidum dimer absorption spectrum Near infra-red spectral atlas Rb2 molecular atlas

a b s t r a c t We present a Doppler-limited transmission spectrum of the rubidium dimer, suitable for frequency calibration of near infrared (e.g. Ti:sapphire) excitation experiments in the region 9500–12 300 cm1. It provides an abundant source of reference peaks that can be used in a graphic environment to calibrate short (<1 cm1) scans of excitation spectrum. This is a sequel to an iodine atlas in ascii format [1] that we routinely use for the same purpose in the visible spectrum. The rubidium spectrum was recorded at an instrumental resolution of 0.018 cm1. Absolute precision is expected to be 0.005 cm1, and relative precision 0.003 cm1. The Rb2 A–X transmission spectrum is available in ascii format, as supplementary material. Ó 2010 Elsevier Inc. All rights reserved.

1. Introduction This work was carried out to provide a secondary wavelength reference spectrum to allow calibration of short scans of excitation spectrum in the region around 1 lm. Molecular atlases already exist for this spectral window. The long wavelength part of the iodine atlas recorded by Gerstenkorn and Luc [2] covers (and indeed extends well beyond) this region, but it was recorded with heated cells (1.5 m long at 350 °C up to 11 000 cm1, and 30 cm long at 790 °C for the region 11 000–13 000 cm1). Such systems are not particularly convenient for routine laboratory use. Amiot and coworkers, also at Laboratoire Aimé Cotton, recognized this and addressed the problem some years ago by recording three separate atlases of potassium, rubidium and caesium dimers, making line lists and photographic records available [3–5]. They noted that not only were the experimental conditions easier to manage, but also that for all three alkalis, the influence of hyperfine structure was much smaller than for I2, allowing better wavenumber determinations. Their line lists were deliberately and carefully restricted to include nearly resolved, symmetrical spectral features, to minimize problems with peak-finding algorithms. This limited the number of reference wavenumbers available. One major difference between the absorption spectra of I2 and Rb2 is that lines in the iodine spectrum are for the most part resolved, whereas those of Rb2 are not. The I2 line lists in this wavelength region are restricted to resolved and symmetric features, so data are sometimes sparse in a 1 cm1 window. The rubidium spectrum is extremely crowded, however, because of its smaller vibrational constants, because 85Rb2 and 85Rb87Rb contribute nearly equally to the spectrum, and because additional b ⇑ Corresponding author. Fax: +33 4 72 44 58 71. E-mail addresses: [email protected], [email protected] (A.J. Ross). 0022-2852/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jms.2010.09.008

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Pu  X 1 Rþg transitions appear due to strong A, b spin–orbit mix-

ing [6]. Even in the case of the lighter sodium dimer, with only one isotope and with much smaller spin–orbit mixing, the Doppler limited A–X spectrum is not properly resolved: a small section of sub-Doppler excitation spectrum recorded by the Lyyra group in Philadelphia was used to illustrate this in Fig. 4 of Ref. [7]. Almost none of the features in the A–X system of Rb2 (or of K2 or Cs2) correspond to individual, resolved lines, but the spectral coverage is complete. By matching contours rather than well-defined peaks, the spectrum provides a practical secondary standard. This work therefore complements the alkali atlases recorded at Laboratoire Aimé Cotton, in the sense that we supply the entire spectrum, rather than a list of peaks corresponding to the symmetricallyshaped lines in the spectrum. By running the heatpipe at slightly higher temperatures we generated sufficient molecular vapour to be able to work with a single-pass experiment, and we also created a thermal population in the electronic ground state such that the spectral coverage from the Rb heatpipe is actually wider than was given by the three alkali atlases from Orsay. 2. Experimental details Strong A 1 Rþ X 1 Rþ u g absorption transitions in the alkali dimers fall in the 800–1000 nm region, so that an absorption spectrum can be recorded in a single pass of white light through a reasonably compact (l = 50 cm) heatpipe source. A ? X fluorescence is likewise strong, and extends to longer wavelengths, so a total fluorescence signal can be readily taken on a Si photodiode, using an optical filter to reduce or remove laser scatter, if necessary. Rubidium vapour was produced in a stainless steel heatpipe oven, overall length 50 cm, internal diameter 3 cm. A stainless steel mesh lined the interior. The central section, 20 cm long, was maintained at 400 °C (the temperature was measured on a

A.J. Ross et al. / Journal of Molecular Spectroscopy 264 (2010) 78–81

thermocouple touching the outer wall of the heatpipe). The heatpipe was filled with a 1 g ampoule of 99% purity rubidium from Aldrich, adding 15 mbar of argon as a buffer gas. A fan was used to air-cool the region immediately outside the oven, allowing metal vapour to condense close to the hot zone of the heatpipe. A water cooling circuit produced a cold region close to the Brewster angle windows. Light from a 100 W quartz halogen lamp was collimated to optimize overlap with rubidium vapour inside the heatpipe, and then focused onto the entrance aperture of a Bomem DA3 Fourier transform spectrometer. The spectrum was recorded in two parts so that the source could be operated at around 80 W without saturating the spectrometer’s Si avalanche detector. The first section was recorded with a high pass k > 900 nm filter (Corion LL900). The lower limit combined the response curve of the detector, and the absorption spectrum of rubidium vapour. The second section was recorded with the white light passing through one high pass k > 800 nm (Corion LL800) and one low-pass k < 950 nm (Corion LL950) filter, ensuring considerable overlap in the strongest region of the spectrum. Background spectra were recorded with the heatpipe at room temperature. All four spectra were recorded at a nominal instrumental resolution of 0.018 cm1, accumulating 100 scans for the shorter wavelength region, and 200 for the longer wavelengths. Background spectra accumulated 250 scans, with the heatpipe at room temperature. The spectra were then spliced

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together, averaging the transmission signals in the region of overlap. The frequency axis was brought to match the calibration of the Aimé Cotton atlas by applying a constant offset. The origin of this systematic shift, of the order of 0.015 cm1 between measurements made under vacuum on these two interferometers, is hard to pinpoint. Although our Fourier transform instrument is internally calibrated with an a priori well-known reference wavelength, it does not generate spectra with equivalent absolute precision. Aligning light from external sources requires some care to respect the instrument’s optical alignment and aperture, otherwise calibration shifts proportional to wavenumber are introduced. Despite our precautions, we still find a considerable constant difference in absolute wavenumber with respect to what we consider to be a reliable source, the spectrum of Ref. [4], where absolute uncertainty was conservatively quoted as 0.005 cm1 in the region of interest; Dr/r, <5  107. This value was the sum of (a) the difference of less than 0.003 cm1 found between the Cs D1 atomic transition measured (independently) at Orsay, and a literature value for its hyperfine centroid, 11178.2681607 (14) cm1 (determined from femtosecond frequency comb experiments [8]), and (b) an uncertainty of 0.002 cm1 from an assessment of instrumental limitations in the Aimé Cotton FT interferometer. The step-by-step interferometer at Laboratoire Aimé Cotton [9] was referenced to superradiant emission from a xenon lamp, whose (Ritz) vacuum

Fig. 1. Overview of the region covered by the rubidium dimer absorption spectrum.

Fig. 2. Illustration of match between successive records. Upper trace: spectrum recorded with 800 nm < k and 950 nm > k filters, lower trace recorded with a 900 nm < k filter. Most of the small features are reproduced in the region of overlap between the two spectra. Filled circles indicate lines listed in the Aimé Cotton atlas (Ref. [4]).

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Fig. 3. Example of an excitation spectrum (lines in the F–X 1–0 band of FeH) calibrated using Rb2 fluorescence and the present atlas. Trace a: Rb2 excitation spectrum recorded on a Si detector as a cw laser is scanned. Trace b: Inverted Rb atlas [1–(transmission)]. Trace c: The FeH spectrum to be calibrated. Trace d: Fabry-Pérot markers (FSR = 0.05 cm1). Arrows indicate the start of successive laser scans. Circles again indicate the lines listed in Ref. [4].

wavelength was cited [10] as 35079.84664 Å (sampling at kref/100). The spectrometer manual for the continuous scanning DA3 instrument used in Lyon quotes a lower absolute accuracy (limited by beam position) of ±0.002 cm1 at 4000 cm1, with a contribution of ±0.001 cm1 at 2000 cm1 taking the stability of the reference laser into account. (The wavelength reference comes from a single mode frequency stabilized HeNe laser, with kref = 6329.9 Å in vacuum, sampling at kref/4). The DA3 value for Dr/r of 106 suggests we should be able to achieve an absolute instrumental precision of ±0.01 cm1 in the region of our atlas; the shift we observe is not far in excess of this figure. The outline of the rubidium transmission spectrum is illustrated in Fig. 1. Fig. 2 shows two successive records of a smaller region, demonstrating that most of the smaller features are reproducible, and not noise. Fig. 3 illustrates an example of calibration with this atlas around 885 nm. About 25 mW of the output from a CR 899 laser was used to excited fluorescence in rubidium dimer, sending the total fluorescence signal through a high pass k > 1000 nm optical filter to a Si-photodiode detector (Thorlabs PDA36A). This excitation spectrum was readily matched to our atlas to calibrate the fragment of FeH spectrum recorded concurrently with the main

part of the laser beam. This 1 cm1 section of spectrum had to be recorded in three pieces. Each piece was referenced to five or more features in the rubidium transmission spectrum, and corrected for non-linear scanning of the Ti:sapphire laser used. Merge points are indicated by vertical arrows in Fig. 3. 3. Comments Having used the Aimé Cotton rubidium atlas as a secondary standard, and having recorded very comparable spectra, we expect the relative and absolute precision of this spectrum to match that work. Ref. [4] gave a conservative estimate of 0.005 cm1 for absolute wavenumber uncertainty. We estimate the relative precision to be 0.003 cm1, because of the ill-resolved nature of the spectrum. This value also reflects the spread of points illustrated in the scattergram shown in Fig. 4, which shows a plot of the difference between a selection of peaks measured on the present spectrum and peaks listed in Ref. [4]. Obtaining a wavenumber scale with absolute precision better than 0.005 cm1 directly from the Fourier transform spectrometer in the visible/near infrared spectrum is not trivial, and in practice, we can only advocate taking a reference spectrum when possible to verify calibration a posteriori. The Cs 6p 2P1/2 6s 2S1/2 line was seen in our spectrum because Cs is present as an impurity. Unfortunately, this line is saturated (see Fig. 1) and so broad (fwhm 0.19 cm1) that it could not be used as a direct reference point for an absolute wavenumber scale. We did however check our final wavenumber scale against atmospheric water lines, clearly visible in the background spectrum. These, too, are broad, and would not have been suitable to establish a frequency shift with confidence, but the line positions match data from the HITRAN data base. NB: these lines are not visible in the final transmission spectrum, because they are present in both the Rb2 absorption scans and the background spectrum. Acknowledgments

Fig. 4. Scattergram of differences between wavenumbers measured on this atlas and wavenumbers taken from the list given in Ref. [4].

We are pleased to acknowledge financial support from the French Agence National pour la Recherche (Grant ANR 08 BLAN0017). We thank Mr. Thomas Lacroix for help with recording the uncalibrated FeH spectrum at 890 nm.

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Appendix A. Supplementary data The rubidium vapour transmission spectrum is given as supplementary material, and can be obtained in ascii format from the Journal of Molecular Spectroscopy Supplementary Material Archive, at the web address http://library.osu.edu/sites/msa/ jmsa_hp.htm, or from the authors, on request. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jms.2010.09.008.

[4]

[5]

[6]

[7]

References [1] H. Salami, A.J. Ross, J. Mol. Spectrosc. 233 (2005) 157–159. [2] S. Gerstenkorn, P. Luc, J. Vergès, Atlas du spectre d’absorption de la molécule d’iode 7220–11200 cm1, Laboratoire Aimé Cotton, CNRS II, Orsay, 1993. [3] C. Amiot, J. Chevillard, J. Vergès. Atlas du spectre d’absorption de la molécule de potassium (10487–12000 cm1). Laboratoire Aimé Cotton, C.N.R.S., Orsay,

[8] [9] [10]

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France, (2000), Available from: . C. Amiot, J. Chevillard, J. Vergès. Atlas du spectre d’absorption de la molécule de rubidium (9992–11715 cm1). Laboratoire Aimé Cotton, C.N.R.S., Orsay, France, (2000), Available from: . C. Amiot, J. Chevillard, J. Vergès. Atlas du spectre d’absorption de la molécule de césium (9700–11000 cm1). Laboratoire Aimé Cotton, C.N.R.S., Orsay, France, (2000), Available from: http://www.lac.u-psud.fr/atlas/Cs2/ intro_Cs2.html. H. Salami, T. Bergeman, B. Beser, J. Bai, E.H. Ahmed, S. Kotochigova, A.M. Lyyra, J. Huennekens, C. Lisdat, A.V. Stolyarov, O. Dulieu, P. Crozet, A.J. Ross. Phys. Rev. A (Atomic, Molecular, and Optical Physics) 80 (2009) 022515–022514. P. Qi, J. Bai, E. Ahmed, A.M. Lyyra, S. Kotochigova, A.J. Ross, C. Effantin, P. Zalicki, J. Vigue, G. Chawla, R.W. Field, T.J. Whang, W.C. Stwalley, H. Knöckel, E. Tiemann, J. Shang, L. Li, T. Bergeman, J. Chem. Phys. 127 (2007) 044301. T. Udem, J. Reichert, R. Holwarth, T.W. Hänsch, Phys. Rev. Lett. 82 (1999) 3568– 3571. J. Connes, H. Delouis, P. Connes, G. Guelachvili, J.P. Maillard, G. Michel, Nouv. Rev. Opt. Appliquée 1 (1970) 3–22. J.G. Conway, J. Blaise, J. Vergès Spectrochim. Acta 31B (1975) 31–47.