Free Access
Issue
A&A
Volume 545, September 2012
Article Number A61
Number of page(s) 10
Section Atomic, molecular, and nuclear data
DOI https://doi.org/10.1051/0004-6361/201219852
Published online 07 September 2012

© ESO, 2012

1. Introduction

The observation of lithium spectral lines in stellar atmospheres provide the astrophysicists with important information about primordial (Big Bang) nucleosynthesis, the evolution, mixing, and spallation processes in stars, stellar rotation history, and even the formation of planets (Lambert 1993; Carlsson et al. 1994; Iocco et al. 2009; Meléndez et al. 2010).

The identification of the main source of Li in the Galaxy (primordial nucleosynthesis, stars, or Galactic cosmic rays) remains unclear (Prantzos 2012). The phenomenon of “lithium depletion” (the so-called “cosmological lithium problem”, see, e.g., Sbordone et al. 2010; Monaco et al. 2010) observed in Sun-like stars, consists in a disagreement between the primordial lithium abundance predicted from standard Big Bang nucleosynthesis and its abundance corresponding to the Spite plateau (Spite & Spite 1982a,b).

It is assumed that the lithium depletion is connected with mixing the different temperature layers by diffusion, meridional circulation, and internal gravity waves (Pace et al. 2012). Some models claim a rotational mixing mechanism and predict stronger Li depletion for planet-hosting stars (Bouvier 2008; Israelian et al. 2009; Eggenberger et al. 2012). Thus, the Li abundance is an effective indicator of mixing processes in stars and therefore spectroscopic observations of Li in metal-poor stars (including relatively old solar-type stars) are needed (Canto Martins et al. 2011; Pace et al. 2012; Nissen & Schuster 2012).

Table 1

Observed new lines of Li I and their identifications.

For cool low-mass objects, the spectroscopic detection of lithium can be applied to determine their mass or age (Yee & Jensen 2010). Recent high-resolution spectroscopic studies have shown lithium enhancements in the atmospheres of some young cool stars that have been classified as lithium-rich K- and M-type giants (Alcalá et al. 2011) and some dwarfs (Koch et al. 2011; Monaco et al. 2012).

Cool objects such as dwarfs, disks, or planets and the extended atmospheres of evolved stars are extensively being studied by infrared (IR) astronomy (Lyubchik et al. 2004, 2007; Kerber et al. 2009). Though the current space-born IR spectrographs (e.g., Herschel, Spitzer, AKARI) have a spectral resolution R ~ 100–1000), the forthcoming spatial and airborne (SOFIA, SPICA) telescopes are expected to have a much higher resolution. The great advantages of Fourier transform infrared (FTIR) spectroscopy, such as its constant high resolution and energy throughput, have made the IR spectral region more accessible for laboratory spectral measurements (Nilsson 2009). However, the powerful capacities of IR astronomy cannot be fully utilized without detailed spectroscopic information on atomic line features (in particular, wavelengths and oscillator strengths) in the IR region (Biémont 1994; Johansson 2005; Pickering et al. 2011; see the references in Civiš et al. 2012a, for more references on IR space- and ground-based astronomy).

The majority of Li abundance studies are based on the analysis of the resonance Li I 670.8 nm line only, while some other lines, e.g., the 610.4 nm line (Merchant 1967; Bonifacio & Molaro 1998) or the 812.6 nm line (Yakovina et al. 2003) can be potentially good lithium-abundance indicators. The most comprehensive, to the authors’ knowledge, Li I line list to date was presented by Radziemski et al. (1995); it covers the 1800−31 000 cm-1 (5.6–3.2 micron) range. No lines with wavelengths longer than 5.6 micron have been reported in the literature.

The most prominent lines in atomic metal spectra below 1800 cm-1 are typically due to the radiative transitions between the atomic states with high orbital momentum l ≥ 4. However, except for the 5g term measured by Litzen (1970), the high-l levels, i.e. the ng (with n > 5) or nh states, have never been observed for Li I.

The present work aims to fill this gap in the available Li I level and line lists. We measure the emission spectra of Li I in the 1300–2200 cm-1 range and, from these spectra, extract the energies of the 6g, 6h and 7h levels. We also present the oscillator strengths of the dipole transitions between the observed levels. These f-values are calculated using the quantum defect theory (QDT) technique. This work is a continuation of our previous studies (Civiš et al. 2012a,b).

2. Methods

We study the IR emission spectrum of Li I, which was measured using high-resolution FTIR spectroscopy of the plasma formed by the laser ablation of an LiF target. The sample surface was irradiated by a high-repetition-rate, pulsed, nanosecond ArF laser (λ = 193 nm, laser pulse width 12 ns, frequency 1.0 kHz, output energy 15 mJ) in a vacuum (10-2 Torr). The emission from the plasma was focused into the spectrometer by a ZnSe (127 mm) lens for the 900–1600 cm-1 spectral range and by a CaF2 (100 mm) lens for the 1600–2200 cm-1 range. Two different detectors (MCT and InSb) and two beamsplitters (KBr and CaF2) were used to cover the measured spectral range. The atomic spectra were measured by a Bruker 120 FTIR spectrometer, which was specially modified for time-resolved measurements and calibrated against the internally stabilized HeNe laser. More details on the experimental setup are given in our previous papers (Kawaguchi et al. 2008; Civiš et al. 2010a; Civiš et al. 2010b, 2011c).

Because the g (n > 5) and h levels are absent from the available line lists, we started from the approximate energies of the n′g- and n″ h-states obtained from the Rydberg formula to identify the lines involving these levels. We then refined these level energies using the measured line wavenumbers. In the cases of transitions with close wavenumbers, we compared the oscillator strengths of the lines, which were calculated using single-channel quantum defect theory (QDT). This theory has already been shown to be efficient in calculating the first- (Alcheev et al. 2002) and second-order (Chernov et al. 2005; Akindinova et al. 2009) matrix elements in atoms and molecules. To show that QDT calculations of dipole transition matrix elements are sufficient for our line identification, we compare some QDT-calculated Rb oscillator strengths with experimental and theoretical data available in the literature.

Tables 2 and 3 compare our QDT-calculated f-values with values taken from different sources. In Table 2, we compare our f-values (f × 100) with oscillator strengths obtained from some available experimental or calculated lifetimes. The f-values were extracted from the reported lifetimes in a way similar to that used in our previous work (Civiš et al. 2010b). Table 3 compares our result with the Li I f-values available from NIST ASD (National Institute of Standards and Technology Atomic Spectra Database; Ralchenko et al. 2011), which lists the data from the reviewing paper (Wiese & Fuhr 2009). As in our previous works (Civiš et al. 2010a; Civiš et al. 2010b, 2011b; Civiš et al. 2012a,b), the overall agreement of our QDT calculations with the results available in other sources is quite satisfactory and this agreement show the QDT calculations to be adequate for the identification of the observed transitions.

3. Results and discussion

In their quite comprehensive study of the Li I spectrum, Radziemski et al. (1995) reported 34 lines in the 1829−30 925 cm-1 range. We observed only one line (2149.871 cm-1) not reported by Radziemski et al. (1995). In total, in the range 900−2200 cm-1, we found four lines that had not been previously measured. These are listed in Table 1 together with the other observed lines and their parameters (wavenumbers, full widths at half-maxima, measured intensities, and signal-to-noise ratios) and identification in Table 1. A part of the recorded spectra is shown in Fig. 1, where two 5l′−6l″ transition lines are shown (l′ = 4, l′′ = 5).

thumbnail Fig. 1

A part of Li I emission spectrum with the transitions between the states with n = 5 and 6.

On the basis of the known value of the 5g level, we extract three new levels of Li I, which are presented in Table 1 in the upper level column (without references), together with other measured or calculated values (with references). The 6g, 6h, and 7h levels have never been measured before. We can compare them only with the variational calculations of Chen & Wang (2005). We note that for the 5g level the energy value 39 096.75 cm-1 calculated by Chen & Wang (2005) differs approximately by 1 cm-1 from the measured values of 39 097.941(6) (Radziemski et al. 1995) and 39 097.96 (Litzen 1970). As can be seen from Table 1, our values are also shifted approximately by 1 cm-1 compared to the results of Chen & Wang (2005). Given the above considerations, we consider our values from Table 1 as recommended ones.

Unfortunately, our resolution is not enough to resolve the fine structure of the observed Li I lines. According to (Radziemski et al. 1995), the 4p level has a fine splitting about 0.04 cm-1, while the fine splitting of the high-l levels should be much more lower. Thus, the j values are not specified in Table 1 and the presented values are those averaged over the multiplets.

Table 2

Comparison of Li I oscillator strengths (f × 100) with strengths obtained from previously published experimental or calculated lifetimes.

Given the above-mentioned agreement of our QDT calculation with the dipole transition matrix elements, we present in Table 4 the matrix elements (f- and A-values) for the transition involving the nd, nf, ng, and nh states of Li I. In particular, this A-value list was used to identify the observed lines according to their relative intensities.

4. Conclusion

This work continues a series of FTIR spectroscopy studies of IR spectra of metal atoms (Civiš et al. 2010a; Civiš et al. 2010b, 2011c,a; Civiš et al. 2012a,b). We have reported the results of laboratory measurements for four new Li I lines in the 900−2200 cm-1 range. To our knowledge, there are no laboratory-measured Li I spectra below 1800 cm-1 (above 5.6 microns). The classification of the lines was performed by accounting for oscillator strengths (f-values) calculated using quantum defect theory (QDT). The comparison of the QDT calculations with the available experimental and theoretical results proves that QDT is an adequate tool for calculating dipole transition matrix elements. The recorded spectra have allowed us to extract the excitation energies of the 6g, 6h, and 7h states of Li I, which had not been measured before.

Table 3

Comparison of oscillator strengths (f-values) for several transitions in Li I with known values from the compilation by Wiese & Fuhr (2009).

Table 4

QDT-calculated oscillator strengths fik and transition probabilities Aki for the transition involving nd, nf, ng, and nh states of Li I.

Acknowledgments

This work was financially supported by the Grant Agency of the Academy of Sciences of the Czech Republic (grant No. IAAX00100903), by the Ministry of Finance of the Czech Republic (Project ECPF:049/4V) and the Ministry of Education, Youth, and Sports of the Czech Republic (grant No. LM2010014).

References

  1. Akindinova, E. V., Chernov, V. E., Kretinin, I. Y., & Zon, B. A. 2009, Phys. Rev. A, 79, 032506 [NASA ADS] [CrossRef] [Google Scholar]
  2. Alcalá, J. M., Biazzo, K., Covino, E., Frasca, A., & Bedin, L. R. 2011, A&A, 531, L12 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  3. Alcheev, P. G., Chernov, V. E., & Zon, B. A. 2002, J. Mol. Spectrosc., 211, 71 [NASA ADS] [CrossRef] [Google Scholar]
  4. Biémont, E. 1994, in Infrared Solar Physics, eds. D. M. Rabin, J. T. Jefferies, & C. Lindsey (Dordrecht, The Netherlands: Kluwer Academic Publ.), IAU Symp., 154, 501 [Google Scholar]
  5. Blundell, S., Johnson, W., Liu, Z., & Sapirstein, J. 1989, Phys. Rev. A, 40, 2233 [NASA ADS] [CrossRef] [Google Scholar]
  6. Bonifacio, P., & Molaro, P. 1998, ApJ, 500, L175 [NASA ADS] [CrossRef] [Google Scholar]
  7. Bouvier, J. 2008, A&A, 489, L53 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  8. Boyd, R. W., Dodd, J. G., Krasinski, J., & Stroud, C. R. 1980, Opt. Lett., 5, 117 [NASA ADS] [CrossRef] [Google Scholar]
  9. CantoMartins, B. L., Lèbre, A., Palacios, A., et al. 2011, A&A, 527, A94 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  10. Carlsson, M., Rutten, R. J., Bruls, J. H. M. J., & Shchukina, N. G. 1994, A&A, 288, 860 [NASA ADS] [Google Scholar]
  11. Chen, C., & Wang, Z.-W. 2005, Commun. Theor. Phys., 43, 886 [NASA ADS] [CrossRef] [Google Scholar]
  12. Chernov, V. E., Dorofeev, D. L., Kretinin, I. Y., & Zon, B. A. 2005, Phys. Rev. A, 71, 022505 [NASA ADS] [CrossRef] [Google Scholar]
  13. Civiš, S., Matulková, I., Cihelka, J., et al. 2010a, Phys. Rev. A, 81, 012510 [NASA ADS] [CrossRef] [Google Scholar]
  14. Civiš, S., Matulková, I., Cihelka, J., et al. 2010b, Phys. Rev. A, 82, 022502 [NASA ADS] [CrossRef] [Google Scholar]
  15. Civiš, S., Kubelík, P., Jelínek, P., Chernov, V. E., & Knyazev, M. Y. 2011a, J. Phys. B, 44, 225006 [NASA ADS] [CrossRef] [Google Scholar]
  16. Civiš, S., Matulková, I., Cihelka, J., et al. 2011b, J. Phys. B, 44, 105002 [NASA ADS] [CrossRef] [Google Scholar]
  17. Civiš, S., Matulková, I., Cihelka, J., et al. 2011c, J. Phys. B, 44, 025002 [Google Scholar]
  18. Civiš, S., Ferus, M., Kubelík, P., Jelinek, P., & Chernov, V. E. 2012a, A&A, 541, A125 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  19. Civiš, S., Ferus, M., Kubelík, P., et al. 2012b, A&A, 542, A35 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  20. Eggenberger, P., Haemmerle, L., Meynet, G., & Maeder, A. 2012, A&A, 539, A70 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  21. Filippov, A. 1931, Z. Phys. A, 69, 526 [CrossRef] [Google Scholar]
  22. Fischer, C., Saparov, M., Gaigalas, G., & Godefroid, M. 1998, ADNDT, 70, 119 [Google Scholar]
  23. Hansen, W. 1983, J. Phys. B, 16, 933 [NASA ADS] [CrossRef] [Google Scholar]
  24. Iocco, F., Mangano, G., Miele, G., Pisanti, O., & Serpico, P. D. 2009, Phys. Rep., 472, 1 [NASA ADS] [CrossRef] [Google Scholar]
  25. Israelian, G., Mena, E. D., Santos, N. C., et al. 2009, Nature, 462, 189 [NASA ADS] [CrossRef] [PubMed] [Google Scholar]
  26. Johansson, S. 2005, in High Resolution Infrared Spectroscopy In Astronomy, Proceedings, eds. H. U. Kaufl, R. Siebenmorgen, & A. Moorwood (Heidelberger, Berlin: Springer-Verlag), ESO Astrophysics Symp., 62 [Google Scholar]
  27. Kawaguchi, K., Sanechika, N., Nishimura, Y., et al. 2008, Chem. Phys. Lett., 463, 38 [NASA ADS] [CrossRef] [Google Scholar]
  28. Kerber, F., Nave, G., Sansonetti, C. J., & Bristow, P. 2009, Phys. Scr., T134, 014007 [NASA ADS] [CrossRef] [Google Scholar]
  29. Koch, A., Lind, K., & Rich, R. M. 2011, ApJ, 738, 29 [Google Scholar]
  30. Lambert, D. L. 1993, Phys. Scr. T, 47, 186 [NASA ADS] [CrossRef] [Google Scholar]
  31. Litzen, U. 1970, Phys. Scr., 1, 253 [NASA ADS] [CrossRef] [Google Scholar]
  32. Lyubchik, Y., Jones, H. R. A., Pavlenko, Y. V., et al. 2004, A&A, 416, 655 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  33. Lyubchik, Y., Jones, H. R. A., Pavlenko, Y. V., et al. 2007, A&A, 473, 257 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  34. McAlexander, W. I., Abraham, E. R. I., Ritchie, N. W. M., et al. 1995, Phys. Rev. A, 51, R871 [NASA ADS] [CrossRef] [PubMed] [Google Scholar]
  35. Meléndez, J., Ramírez, I., Casagrande, L., et al. 2010, Astrophys. Space. Sci., 328, 193 [Google Scholar]
  36. Merchant, A. E. 1967, ApJ, 147, 587 [NASA ADS] [CrossRef] [Google Scholar]
  37. Monaco, L., Bonifacio, P., Sbordone, L., Villanova, S., & Pancino, E. 2010, A&A, 519, L3 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  38. Monaco, L., Villanova, S., Bonifacio, P., et al. 2012, A&A, 539, A157 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  39. Nagourney, W., Happer, W., & Lurio, A. 1978, Phys. Rev. A, 17, 1394 [NASA ADS] [CrossRef] [Google Scholar]
  40. Nilsson, H. 2009, Phys. Scr. T, 134, 014009 [Google Scholar]
  41. Nissen, P. E., & Schuster, W. J. 2012, A&A, 543, A28 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  42. Pace, G., Castro, M., Meléndez, J., Théado, S., & do Nascimento Jr., J.-D. 2012, A&A, 541, A150 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  43. Peach, G., Saraph, H., & Seaton, M. 1988, J. Phys. B, 21, 3669 [NASA ADS] [CrossRef] [Google Scholar]
  44. Pestka, G., & Woznicki, W. 1996, Chem. Phys. Lett., 255, 281 [NASA ADS] [CrossRef] [Google Scholar]
  45. Pickering, J., Blackwell-Whitehead, R., Thorne, A., Ruffoni, M., & Holmes, C. 2011, Can. J. Phys., 89, 387 [NASA ADS] [CrossRef] [Google Scholar]
  46. Prantzos, N. 2012, A&A, 542, A67 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
  47. Qu, L. H., Wang, Z. W., & Guan, X. X. 1997, Chin. Phys. Lett., 14, 732 [NASA ADS] [CrossRef] [Google Scholar]
  48. Qu, L. H., Wang, Z. W., & Li, B. W. 1999, Opt. Commun., 162, 223 [NASA ADS] [CrossRef] [Google Scholar]
  49. Radziemski, L. J., Engleman, R., & Brault, J. W. 1995, Phys. Rev. A, 52, 4462 [NASA ADS] [CrossRef] [Google Scholar]
  50. Ralchenko, Y., Kramida, A., Reader, J., & NIST ASD Team 2011, NIST Atomic Spectra Database, version 4.1.0 [Google Scholar]
  51. Sbordone, L., Bonifacio, P., Caffau, E., et al. 2010, A&A, 522, A26 [Google Scholar]
  52. Spite, F., & Spite, M. 1982a, A&A, 115, 357 [NASA ADS] [Google Scholar]
  53. Spite, M., & Spite, F. 1982b, Nature, 297, 483 [NASA ADS] [CrossRef] [Google Scholar]
  54. Theodosiou, C. E. 1984, Phys. Rev. A, 30, 2881 [NASA ADS] [CrossRef] [Google Scholar]
  55. Volz, U., Majerus, M., Liebel, H., Schmitt, A., & Schmoranzer, H. 1996, Phys. Rev. Lett., 76, 2862 [NASA ADS] [CrossRef] [PubMed] [Google Scholar]
  56. Wiese, W. L., & Fuhr, J. R. 2009, J. Phys. Chem. Ref. Data, 38, 565 [NASA ADS] [CrossRef] [Google Scholar]
  57. Yakovina, L., Pavlenko, Y., & Abia, C. 2003, Astrophys. Space Sci., 288, 279 [NASA ADS] [CrossRef] [Google Scholar]
  58. Yan, Z.-C., & Drake, G. W. F. 1995, Phys. Rev. A, 52, R4316 [NASA ADS] [CrossRef] [PubMed] [Google Scholar]
  59. Yee, J. C., & Jensen, E. L. N. 2010, ApJ, 711, 303 [NASA ADS] [CrossRef] [Google Scholar]

All Tables

Table 1

Observed new lines of Li I and their identifications.

Table 2

Comparison of Li I oscillator strengths (f × 100) with strengths obtained from previously published experimental or calculated lifetimes.

Table 3

Comparison of oscillator strengths (f-values) for several transitions in Li I with known values from the compilation by Wiese & Fuhr (2009).

Table 4

QDT-calculated oscillator strengths fik and transition probabilities Aki for the transition involving nd, nf, ng, and nh states of Li I.

All Figures

thumbnail Fig. 1

A part of Li I emission spectrum with the transitions between the states with n = 5 and 6.

In the text

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