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High 2s2np Rydberg series and ionization energy of B I
Journal of Quantitative Spectroscopy & Radiative Transfer 73 (2002) 121–128 www.elsevier.com/locate/jqsrt
High 2s2np Rydberg series and ionization energy of B I W.L. Glab ∗ , Alan M. Falleur Department of Physics, Texas Tech University, P.O. Box 41051, Lubbock, TX 79409, USA Received 1 March 2001; received in revised form 21 May 2001
Abstract We have used double-resonance excitation of a laser-vaporized sample of atomic boron to measure the energies of np Rydberg states from n∼35 to 70; to a precision of ±0:02 cm−1 . The 2s2 3s state was excited resonantly using light at 249 nm from a frequency-doubled narrowband pulsed dye laser; subsequently, 2s2 np Rydberg states were excited using a second frequency-doubled dye laser operating between 375 and 372 nm. Energy calibration of the spectrum was done by comparison to the laser-induced ;uorescence spectrum of molecular iodine. The Rydberg spectrum was
1. Introduction Despite the fact that it is a light element, the spectrum of highly excited B I has been relatively unexplored by modern laser-based spectroscopic methods. Presumably, this situation has existed due to the diBculty of producing boron in the gas phase, and the fact that the wavelengths required for stepwise excitation lie in the ultraviolet region. Techniques for producing atomic boron for spectroscopy in past studies have included photodissociation of boron containing gaseous compounds [1], pulsed ion beam sputtering from a solid sample [2], dissociation in the plasma of an electric gas discharge [3,4], and pulsed laser ablation or vaporization [5]. Over the decades, a number of studies of the spectrum of boron excited directly from the ground state using conventional light sources and spectroscopic methods have appeared. These have yielded ∗
Corresponding author. Tel.: +1-806-742-3776; fax: +1-806-742-1182. E-mail address: [email protected] (W.L. Glab). 0022-4073/02/$ - see front matter ? 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 2 - 4 0 7 3 ( 0 1 ) 0 0 1 7 7 - 7
122
W.L. Glab, A.M. Falleur / Journal of Quantitative Spectroscopy & Radiative Transfer 73 (2002) 121–128
accurate energies for the low-lying excited states of the atom, as well as the observation of a number of doublet and quartet doubly excited states. Additionally, Rydberg state transitions were measured in several of these works. Edlen et al. [3] quoted an accurate value for the energies of the resonance 2s2 2p–2s2 3s transition doublet, and measured the energies of transitions to nf singly excited Rydberg states with n ranging from 4 to 11. Extrapolation to the series limit gave an ionization energy of 66; 928:10 ± 0:1 cm−1 , the most accurate determination to date. This study also examined the isotope shift and
W.L. Glab, A.M. Falleur / Journal of Quantitative Spectroscopy & Radiative Transfer 73 (2002) 121–128
123
11 B
state is more tightly bound than that of 10 B), with a speci
3. Experiment The experiment was performed in a simple time-of-;ight mass spectrometer. The laser beams used for double-resonance excitation crossed a beam of boron atoms perpendicularly at a location between the
124
W.L. Glab, A.M. Falleur / Journal of Quantitative Spectroscopy & Radiative Transfer 73 (2002) 121–128
shot to shot, and the signal did not degrade noticeably during the time of a spectroscopy scan (∼25 min). The atomic boron was excited by resonant stepwise two-photon excitation through the 2s2 3s 2 S1=2 upper level of the resonance transition. The light required to drive the process was obtained from two homebuilt tunable dye laser=ampli
W.L. Glab, A.M. Falleur / Journal of Quantitative Spectroscopy & Radiative Transfer 73 (2002) 121–128
125
Fig. 1. A portion of the spectrum of transitions from the 2s2 3s intermediate state to high 2s2 np Rydberg states versus photon energy. The vertical scale is proportional to the number of ions detected per laser shot. The range of principal quantum numbers in this scan is from 43 to 69.
achieved by splitting the full spectrum into smaller parts, and individually calibrating those sections. So, we split the spectra into parts of ∼10 cm−1 width, identi
126
W.L. Glab, A.M. Falleur / Journal of Quantitative Spectroscopy & Radiative Transfer 73 (2002) 121–128
Table 1 Observed
11
B 2s2 3s–2s2 np transition energies, cm−1
n
TE; cm−1 (±0:02 cm−1 )
n
TE; cm−1 (±0:02 cm−1 )
30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49
26,762.08 26,770.25 26,777.61 26,784.33 26,790.43 26,796.04 26,801.14 26,805.89 26,810.25 26,814.20 26,817.92 26,821.39 26,824.57 26,827.55 26,830.34 26,832.93 26,835.33 26,837.56 26,839.69 26,841.66
50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69
26,843.52 26,845.30 26,846.96 26,848.52 26,850.01 26,851.41 26,852.73 26,853.99 26,855.17 26,856.28 26,857.34 26,858.37 26,859.36 26,860.31 26,861.14 26,862.00 26,862.78 26,863.55 26,864.28 26,865.02
widths averaged a little over 0:1 cm−1 in the frequency doubled photon energy, and therefore only involved a few dye laser steps. We were able to just barely resolve the isotope shift of ∼0:14 cm−1 between the Rydberg transitions of 10 B and the more abundant 11 B; consequently, we measured the large 11 B peaks and disregarded the much smaller peaks of the other isotope (which are barely visible as red shoulders on some of the Rydberg resonances), and made no correction to the 11 B peak energies due to the presence of 10 B. This can be justi
(Ry)11B ; (n − np )2
where (Ry)11B is the Rydberg constant for 11 B; 10;9731:74 cm−1 ; I(2s2 3s) is the energy required to ionize the intermediate state, np is the quantum defect for the 2s2 np series, and the latter two of these parameters were varied. The best
W.L. Glab, A.M. Falleur / Journal of Quantitative Spectroscopy & Radiative Transfer 73 (2002) 121–128
127
Fig. 2. The diOerences between the measured transition energies and those calculated from the best
between the measured Rydberg transition energies and the values calculated from the
128
W.L. Glab, A.M. Falleur / Journal of Quantitative Spectroscopy & Radiative Transfer 73 (2002) 121–128
however, it is only in marginal agreement with their theoretical value of 0.51. Since no precision level was quoted for their theoretical result, this sort of disagreement is not disturbing. 6. Conclusion Laser vaporization of elemental boron provides a means of producing a stable and suBciently dense atomic beam for high resolution excited state spectroscopy using double resonance. Our double resonance spectra show that the 2s2 np Rydberg states of boron are unperturbed at our level of precision, and can be used for an accurate determination of the ionization energy of the atom. Our result, 66;928:06 ± 0:03 cm−1 , is in agreement with previous results, but has considerably higher precision. A signi
High 2s2np Rydberg series and ionization energy of B I W.L. Glab ∗ , Alan M. Falleur Department of Physics, Texas Tech University, P.O. Box 41051, Lubbock, TX 79409, USA Received 1 March 2001; received in revised form 21 May 2001
Abstract We have used double-resonance excitation of a laser-vaporized sample of atomic boron to measure the energies of np Rydberg states from n∼35 to 70; to a precision of ±0:02 cm−1 . The 2s2 3s state was excited resonantly using light at 249 nm from a frequency-doubled narrowband pulsed dye laser; subsequently, 2s2 np Rydberg states were excited using a second frequency-doubled dye laser operating between 375 and 372 nm. Energy calibration of the spectrum was done by comparison to the laser-induced ;uorescence spectrum of molecular iodine. The Rydberg spectrum was
1. Introduction Despite the fact that it is a light element, the spectrum of highly excited B I has been relatively unexplored by modern laser-based spectroscopic methods. Presumably, this situation has existed due to the diBculty of producing boron in the gas phase, and the fact that the wavelengths required for stepwise excitation lie in the ultraviolet region. Techniques for producing atomic boron for spectroscopy in past studies have included photodissociation of boron containing gaseous compounds [1], pulsed ion beam sputtering from a solid sample [2], dissociation in the plasma of an electric gas discharge [3,4], and pulsed laser ablation or vaporization [5]. Over the decades, a number of studies of the spectrum of boron excited directly from the ground state using conventional light sources and spectroscopic methods have appeared. These have yielded ∗
Corresponding author. Tel.: +1-806-742-3776; fax: +1-806-742-1182. E-mail address: [email protected] (W.L. Glab). 0022-4073/02/$ - see front matter ? 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 2 - 4 0 7 3 ( 0 1 ) 0 0 1 7 7 - 7
122
W.L. Glab, A.M. Falleur / Journal of Quantitative Spectroscopy & Radiative Transfer 73 (2002) 121–128
accurate energies for the low-lying excited states of the atom, as well as the observation of a number of doublet and quartet doubly excited states. Additionally, Rydberg state transitions were measured in several of these works. Edlen et al. [3] quoted an accurate value for the energies of the resonance 2s2 2p–2s2 3s transition doublet, and measured the energies of transitions to nf singly excited Rydberg states with n ranging from 4 to 11. Extrapolation to the series limit gave an ionization energy of 66; 928:10 ± 0:1 cm−1 , the most accurate determination to date. This study also examined the isotope shift and
W.L. Glab, A.M. Falleur / Journal of Quantitative Spectroscopy & Radiative Transfer 73 (2002) 121–128
123
11 B
state is more tightly bound than that of 10 B), with a speci
3. Experiment The experiment was performed in a simple time-of-;ight mass spectrometer. The laser beams used for double-resonance excitation crossed a beam of boron atoms perpendicularly at a location between the
124
W.L. Glab, A.M. Falleur / Journal of Quantitative Spectroscopy & Radiative Transfer 73 (2002) 121–128
shot to shot, and the signal did not degrade noticeably during the time of a spectroscopy scan (∼25 min). The atomic boron was excited by resonant stepwise two-photon excitation through the 2s2 3s 2 S1=2 upper level of the resonance transition. The light required to drive the process was obtained from two homebuilt tunable dye laser=ampli
W.L. Glab, A.M. Falleur / Journal of Quantitative Spectroscopy & Radiative Transfer 73 (2002) 121–128
125
Fig. 1. A portion of the spectrum of transitions from the 2s2 3s intermediate state to high 2s2 np Rydberg states versus photon energy. The vertical scale is proportional to the number of ions detected per laser shot. The range of principal quantum numbers in this scan is from 43 to 69.
achieved by splitting the full spectrum into smaller parts, and individually calibrating those sections. So, we split the spectra into parts of ∼10 cm−1 width, identi
126
W.L. Glab, A.M. Falleur / Journal of Quantitative Spectroscopy & Radiative Transfer 73 (2002) 121–128
Table 1 Observed
11
B 2s2 3s–2s2 np transition energies, cm−1
n
TE; cm−1 (±0:02 cm−1 )
n
TE; cm−1 (±0:02 cm−1 )
30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49
26,762.08 26,770.25 26,777.61 26,784.33 26,790.43 26,796.04 26,801.14 26,805.89 26,810.25 26,814.20 26,817.92 26,821.39 26,824.57 26,827.55 26,830.34 26,832.93 26,835.33 26,837.56 26,839.69 26,841.66
50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69
26,843.52 26,845.30 26,846.96 26,848.52 26,850.01 26,851.41 26,852.73 26,853.99 26,855.17 26,856.28 26,857.34 26,858.37 26,859.36 26,860.31 26,861.14 26,862.00 26,862.78 26,863.55 26,864.28 26,865.02
widths averaged a little over 0:1 cm−1 in the frequency doubled photon energy, and therefore only involved a few dye laser steps. We were able to just barely resolve the isotope shift of ∼0:14 cm−1 between the Rydberg transitions of 10 B and the more abundant 11 B; consequently, we measured the large 11 B peaks and disregarded the much smaller peaks of the other isotope (which are barely visible as red shoulders on some of the Rydberg resonances), and made no correction to the 11 B peak energies due to the presence of 10 B. This can be justi
(Ry)11B ; (n − np )2
where (Ry)11B is the Rydberg constant for 11 B; 10;9731:74 cm−1 ; I(2s2 3s) is the energy required to ionize the intermediate state, np is the quantum defect for the 2s2 np series, and the latter two of these parameters were varied. The best
W.L. Glab, A.M. Falleur / Journal of Quantitative Spectroscopy & Radiative Transfer 73 (2002) 121–128
127
Fig. 2. The diOerences between the measured transition energies and those calculated from the best
between the measured Rydberg transition energies and the values calculated from the
128
W.L. Glab, A.M. Falleur / Journal of Quantitative Spectroscopy & Radiative Transfer 73 (2002) 121–128
however, it is only in marginal agreement with their theoretical value of 0.51. Since no precision level was quoted for their theoretical result, this sort of disagreement is not disturbing. 6. Conclusion Laser vaporization of elemental boron provides a means of producing a stable and suBciently dense atomic beam for high resolution excited state spectroscopy using double resonance. Our double resonance spectra show that the 2s2 np Rydberg states of boron are unperturbed at our level of precision, and can be used for an accurate determination of the ionization energy of the atom. Our result, 66;928:06 ± 0:03 cm−1 , is in agreement with previous results, but has considerably higher precision. A signi