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Radiative lifetimes of odd-parity levels in Nb I
Journal of Quantitative Spectroscopy & Radiative Transfer 166 (2015) 68–73
Contents lists available at ScienceDirect
Journal of Quantitative Spectroscopy & Radiative Transfer journal homepage: www.elsevier.com/locate/jqsrt
Radiative lifetimes of odd-parity levels in Nb I Sheo Mukund a,b, Soumen Bhattacharyya a, Suresh Yarlagadda b, S.G. Nakhate a,b,n a b
Atomic and Molecular Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India Homi Bhabha National Institute, Bhabha Atomic Research Centre, Mumbai 400085, India
a r t i c l e in f o
abstract
Article history: Received 26 May 2015 Received in revised form 24 July 2015 Accepted 25 July 2015 Available online 4 August 2015
Radiative lifetimes are reported for 37 odd-parity energy levels of neutral niobium (Nb I), out of which 33 have been measured for the first time. The levels belong to electronic configurations 4d35s5p and 4d45p between 18,790 and 35,730 cm 1. The time-resolved laser-induced fluorescence spectroscopy technique was employed. The Nb atoms were generated in a free-jet by laser vaporization of niobium metal. Lifetime values reported in this work fall in the range 12–340 ns and are accurate to 710%. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Atomic spectra Radiative lifetime Branching fraction Time-resolved laser-induced fluorescence
1. Introduction Accurate radiative lifetimes of the excited atomic/ionic energy levels are of immense interest. The radiative lifetimes combined with accurate branching fractions, provide one of the most reliable methods for determining the absolute transition probabilities of spectral lines. Corliss and Bozman [1] have reported transition probabilities of spectral lines of many refractory metals based on the absolute emission measurements on intense arcs. However, the use of this data for determination of the solar abundance raises some doubts on their accuracy [2]. Apart from the basic interest in the radiative lifetimes, the transition probability values are required for numerous applications. The determination of the solar and stellar abundances of the chemical elements requires the knowledge of accurate transition probabilities. Accurate values of transition probabilities of transition metal elements are required in fusion research for the diagnosis of impurities in fusion-oriented plasma. Refractory metals such as niobium are used in the walls of plasma-confinement n Corresponding author at: Atomic and Molecular Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India. Fax: þ 91 22 25505151. E-mail address: [email protected] (S.G. Nakhate).
http://dx.doi.org/10.1016/j.jqsrt.2015.07.019 0022-4073/& 2015 Elsevier Ltd. All rights reserved.
devices. Radiative lifetime values are useful for assessment of the accuracy of theoretical data in atoms with complex electronic structure like transition metals and rare earths. In addition, spectroscopy based elemental analytical methods require knowledge of the oscillator strength. For the above reasons, the knowledge of accurate lifetime data is of continuing interest in atomic physics. The radiative lifetimes of Nb I have been reported in the literature by only four papers. Niobium, being a refractory metal, is difficult to vaporize using conventional thermal means. Several methods, like hollow-cathode effusive atomic beam, atomic beam formed by resistive heating of novel double cage metal wire and by electron bombardment of metal, and recently laser-produced metal plasma were employed to generate niobium atoms in gas phase. Lawler [3] have used time-resolved laser-induced fluorescence (TRLIF) in atomic beam based on hollow-cathode sputtering and measured radiative lifetimes of 50 odd-parity energy levels in the range 19,900–28,000 cm 1. The same group [4] have measured branching fractions of the Nb I spectral lines emerging from these odd-parity energy levels by using a hollow-cathode discharge source coupled to Fourier transform spectrometer and determined the absolute transition probabilities in 320 spectral lines by combining the radiative lifetime data determined earlier [3]. Kwiatkowski et al. [5]
S. Mukund et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 166 (2015) 68–73
2. Experiment The lifetime measurements in Nb I have been carried out in a free-jet apparatus. For details of the experiment readers are referred to read reference [9]. In brief, a rotating and translating niobium metal rod of 99.9% purity was ablated with a third harmonic beam of a Nd:YAG laser with a 10– 15 mJ/pulse energy focused to an 1 mm2 spot. The generated laser produced plasma was cooled and recombined by a supersonic helium gas pulse emanating from a nozzle of a pulse valve at about 210 kPa pressure, into a small channel and expanded freely into a vacuum chamber. Translationally cooled Nb atoms were excited to an energy level of interest, orthogonal to the supersonic expansion axis about 50 mm downstream the nozzle by a pulse dye laser having a pulse duration and bandwidth of 11 ns and 0.1 cm 1, respectively. A time synchronization and required delays between the gas pulse, ablation and excitation laser was achieved by a digital delay generator. Laser-induced fluorescence (LIF) was collected by a biconvex 50 mm f/1 lens along the axis orthogonal to both the atomic and laser beams and imaged on the entrance slit of the monochromator with magnification of 4 and detected by a cooled photomultiplier tube (PMT) having a risetime of 3 ns. The monochromator was used as a broad
band filter by keeping the entrance and exit slits 2.25 mm wide. The fluorescence decay signal detected by the PMT was further amplified by a fast amplifier (1 GHz bandwidth) and stored on a 200 MHz digital storage oscilloscope, triggered by the excitation laser pulse detected by a fast photodiode. The decay signal was averaged for 128 shots in order to obtain a good signal-to-noise ratio before transferring to a personal computer for analysis. 3. Measurements and results We have observed excitation lines of Nb I of reasonably high intensity originating from the ground and the low lying metastable levels up to the energy of 11,524.65 cm 1 [10]. The population of the Nb atoms in these metastable states after supersonic expansion was due to excitation processes in the laser-induced plasma. In view of the rich spectra of niobium atom, care was taken to identify correctly the transitions associated with the excited level of interest. In order to make sure that the correct level was excited; wavenumber calibration of excitation spectra with the accuracy of 0.1 cm 1 was carried out by using Ne optogalvanic spectral lines as well as commercial wavelength meter. In addition to this, the dispersed fluorescence (DF) spectrum was also recorded to confirm the fluorescence decay channels. The excitation line associated with the energy level of interest thus can be identified with high degree of confidence by comparing excitation wavelength and DF lines with spectral line list of Meggers et al. [11]. A few observed excitation and dispersed fluorescence lines not listed in [11], have also been classified by comparing the possible transition wavenumbers computed by using electric dipole selection rules of transitions among listed energy levels of Nb I [10] with observed wavenumbers. Fluorescence decay signals for lifetime determination of a level under investigation were usually recorded in more than one suitable decay channels. In addition to the statistical error, the measured lifetime gets modified due to contribution from the various effects which amounts to the systematic error. The effect of various experimental 0.5
Fluorescence Intensity (arb. units)
employed electron bombardment to generate Nb atomic beam and used TRLIF to measure radiative lifetimes in 11 odd-parity levels of Nb I. They deduced f-values from the measured lifetime data and branching fractions, calculated from Corliss and Bozman [1], and redetermined the photospheric abundance of niobium. Rudolph and Helbig [6] reported a new atomic source suitable for producing beams of refractory elements and measured lifetimes in six levels of niobium atom. Recently, Malcheva et al. [7] reported radiative lifetimes of 17 odd levels in Nb I in the energy range 27,400–47,700 cm 1 using TRLIF method in laser-produced plasma of niobium metal and compared with the theoretical calculations performed using a relativistic Hartree-Fock method including core polarization. By combining the experimental lifetimes and the calculated branching fractions, they obtain transition probabilities for the spectral lines emerging from the investigated levels. From so far published results, it appears that the lifetimes for most of the measured levels are consistent with each other. In spite of these efforts, radiative lifetime data for a number of oddparity levels are still missing. In this paper, we report radiative lifetime measurements on 37 levels of Nb I, out of which 33 are reported for the first time, in the region 18,790–35,730 cm 1 using time-resolved laser-induced fluorescence method in a supersonic free-jet. We have used the technique of laser vaporization in conjunction with supersonic expansion for generating atomic beam of a refractory element, niobium. The supersonic atomic beam allows the excitation of atoms in collision-free environment at low atom number density and thus measured radiative lifetimes are free from collisional and radiation trapping effects. However, because of the high translational velocity associated with the free-jet, this method suffers from the disadvantage of shortening of lifetime due to the flight-outof view effect [8], generally for longer lifetimes of the order of 1 ms or more.
69
0.4
0.3
0.2
0.1
0.0 0
50
100
150
200
250
Time (ns) Fig. 1. A typical fluorescence decay of the 28,079.09 cm 1 level of Nb I (circles). Triangles show recorded excitation laser pulse and the solid curve represents the convolution fit of the laser pulse with an exponential decay function that gave a lifetime value of 30.17 2.7 ns.
70
Table 1 Radiative lifetimes of 37 odd-parity levels of Nb I. Upper levela
Lower levela
Config.
Term
4d35s(5F)5p
Z 6F 0
4d4(a3P)5p
Z 4F 0
Z 2D 0
E (cm
)
J
E (cm
1
)
J
1/2 3/2 5/2 5/2
0.0 1142.79 1586.90 1586.90
532.022c 566.471 572.919 560.352
23,574.14 24,506.53
5/2 3/2 3/2 7/2
391.99 154.19 154.19 695.25
3/2
23,525.80
1/2
5/2
24,773.03
1/2
18,791.09
3/2 5/2
19,036.55 19,427.90
3/2
23,243.87
5/2 9/2
1
)
Lifetime (ns) This work
Previous work
1142.79
566.5
248.0(150)
3/2 7/2
1142.79 2154.11
558.7 578.7
289.5(195) 340.6(221)
437.478 432.973 426.867 419.851
3/2
1142.79
452.3
46.3(14)
44.4d
5/2 7/2 9/2
1586.90 2154.11 2805.36
454.7 447.2 460.7
47.3(6) 37.7(7)
44.5d, 42.7(20)e 35.9d, 33.2(17)e
0.0
424.946
695.25
415.204
4998.17 5297.92 5965.45 154.19 391.99 5965.45
539.6 548.5c 569.3 406.1 410.0 531.5
155.2(27)
7/2
1/2 3/2 5/2 3/2 5/2 5/2 3/2 1/2
5297.92 8410.90
537.1c 645.0c
71.5(31)
7/2
695.25
388.3
38.0(10)
5/2 7/2 5/2
1586.90 2154.11 5965.45
387.9 396.6 467.3c
40.7(47)
365.598 364.985 356.362 353.366
3/2 5/2 5/2
5297.92 5965.45 5965.45
450.3 458.2 444.7
22.9(11) 31.1(15) 16.0(10)
Z 2S 0
1/2
23,910.90
3/2
154.19
420.816
4d4(5D)5p
y 4F 0
9/2
26,440.33
9/2
1050.26
393.744
4
E (cm
λfluo (nm)b
3/2
4d4(a3P)5p
4 5
Lower level in fluorescence
4d ( D)5p
y D
0
5/2
27,359.70
3/2
4d35s(c3P)5p
y 4p 0
1/2 3/2 5/2
27,498.94 27,782.57 28,445.33
3/2 5/2 5/2 3/2
–
276o
5/2
27,614.10
3/2
154.19
364.064
7/2 5/2 7/2
695.25 1586.90 2154.11
371.4 384.1c 392.7
101.0(50)
4d35s(c3P)5p
x 4D 0
1/2
27,666.46
3/2
154.19
363.371
28,079.09
5/2
28,549.42
3/2 1/2 5/2
154.19 0.0 391.99
358.006c 356.040c 355.045
7/2
29,209.42
5/2 5/2
391.99 5965.45
346.918c 430.099
0.0 1142.79 5297.92 391.99 5297.92 695.25 5297.92 5965.45 695.25 2805.36 5965.45
361.3c 376.9 446.9 361.1c 438.8 358.9 430.0 442.7 350.6c 378.6 430.1
22.4(21)
3/2
1/2 3/2 3/2 5/2 3/2 7/2 3/2 5/2 7/2 9/2 5/2
154.19
154.19 391.99 391.99 154.19
367.472
c
64.6(8)
30.1(27) 15.7(7)
30.7(17)
4d35s(a3F)5p
z 2F 0
7/2
28,535.36
7/2
695.25
359.096c
7/2 5/2
2154.11 5965.45
378.9 442.9
123.9(72)
4d35s(a3F)5p
y 2G 0
9/2
28,433.74
9/2
1050.26
365.081
7/2
2154.11
380.4c
105.7(74)
35.6d
S. Mukund et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 166 (2015) 68–73
4d35s(a3F)5p
J
1
λexc. (nm)b
2
–
z p
0
3/2
27,918.85
1/2
0.0
358.083
c
c
354.402
5/2 1/2 5/2
1586.90 4998.17 5965.45
375.5 430.7c 449.4
12.3(6)
7/2 5/2 9/2
2154.11 5965.45 2805.36
362.1 420.1 366.0
45.5(17)
7/2
29,762.70
7/2
2154.11
362.103
9/2
30,117.32
9/2
1050.26
343.938c
1142.79
c
4d4(a1D)5p
x 4F 0
y 2F 0
4d4(3D)5p
v 4F 0
4d35s(1G)5p
x 2G 0
4d4(a1G)5p
w 2F 0
4d4(3H)5p a
4 0
I
3/2
29,622.73
3/2
351.029
c
38.1(14) c
1/2 3/2 3/2
4998.17 5297.92 5297.92
406.0 411.0 408.4
57.8(25)
3/2 5/2 7/2 5/2 5/2 9/2
1142.79 391.99 695.25 1586.90 1586.90 2805.36
349.1 337.8c 341.3c 352.0c 349.9c 365.4c
41.7(23) 34.4(11)
3/2 7/2 3/2 5/2 7/2
1142.79 2154.11 5297.92 1586.90 2154.11
338.0 350.0c 393.3c 339.6 346.3
47.0(17)
5/2
29,775.80
5/2
1586.90
354.653
3/2 5/2
29,779.44 29,987.45
5/2 3/2
1586.90 1142.79
354.607c 346.586
7/2
30,161.56
7/2
2154.11
356.947
5/2
30,716.50
5/2
1586.90
343.200c
7/2
31,025.52
5/2
1586.90
339.593
7/2
31,973.24
9/2
2805.36
342.745
9/2 5/2 5/2
2805.36 5965.45 9043.14
342.7 384.4 436.0
33.3(7)
32,213.94
9/2 9/2
2805.36 9328.88
339.940 436.843
9/2
2805.36
339.9
46.2(27)
7/2
32,087.58
9/2
2805.36
341.407
5/2 9/2 5/2
1586.90 12,102.12 13,404.77
327.8 500.2 535.1c
22.9(12)
15/2
35,731.34
13/2
11,524.65
412.993f
13/2
11,524.65
413.0
18.7(11)
9/2
Kramida et al. [10]. Wavelengths in air from Meggers et al. [11] except those marked with superscript “c.” c Transition wavelengths in air, not listed in Meggers et al. [11]. These are classified in this work and confirmed by dispersed fluorescence. d Duquette et al. [3]. e Kwiatkowski et al. [5]. f Classification by Kröger et al. Eur Phys J D 2007;41:61–70. b
66.1(44)
98.0(90)
32.4(14)
70.3(20)
S. Mukund et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 166 (2015) 68–73
y 4G 0
4d4(b3F)5p
29.3(18)
0.0
4d4(3H)5p
y D
436.2c 455.4 366.2c 426.4c 431.9c
351.495
1/2
4d 5s(a F)5p
4998.17 5965.45 1142.79 4998.17 5297.92
0.0
28,208.48
0
1/2 5/2 3/2 1/2 3/2
1/2
3/2
2
390.1c
28,442.16
z 4S 0
3
2805.36
1/2
4d4(a3P)5p
3
9/2
71
72
S. Mukund et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 166 (2015) 68–73
parameters on the systematic variation of the measured lifetime of a level was studied. Care was taken to minimize the influence on the lifetime measurements due to various effects like flight-out-of-view, radiation trapping, saturation, and collisional depopulation. The flight-out-of-view effect is a major spoiler, especially for longer lifetimes; in a free-jet setup and it was investigated rigorously in the present and our previous works [9,12]. The contribution from this effect was investigated by recording long lifetimes of the order of 300–400 ns at different delay time between the ablation and excitation lasers and also at lower backing pressures up to 140 kPa. Longer delay times probe the slower atoms than those excited at short delay time. Low backing pressure reduces the average atomic beam velocity. No obvious systematic variation in the lifetimes was observed with the variation in the delay and backing pressure, at least to the longest lifetime reported in the present work. The contributions of saturation and radiation trapping effects to the measured lifetimes were also investigated by measuring the lifetimes under different excitation laser pulse energies and number densities of Nb atoms. The Nb atom number density was varied by changing the ablation laser intensity. The lifetime values determined for a level with the variation in excitation and ablation laser intensity were also scattered randomly around a mean value. The error in the measurement due to nonlinear response of the photomultiplier tube at high fluorescence intensities was minimized by keeping the fluorescence intensity low either by inserting neutral density filters in front of the entrance slit of the monochromator or by exciting a transition with a low laser pulse energy. Since the present work was carried out in the supersonic free-jet operating at low base pressure o10 7 mbar, the lifetime reduction due to collisional depopulation was insignificant. The time resolved decay curve may exhibit a modulation at the hyperfine splitting frequencies. Niobium has only one naturally abundant isotope, 93Nb and it has the largest nuclear magnetic dipole moment among the stable nuclei. Thus the atomic levels of Nb have a broad hyperfine splitting [13,14]. As the frequency spread (1/Δτ 100 MHz) corresponding to the duration of the excitation pulse cannot cover the large hyperfine splitting, which is of the order of few hundreds of MHz, pairs of nondegenerate magnetic sublevels cannot be coherently excited [15]. Thus the systematic error due to modulations in the fluorescence decay signal due to hyperfine effect is insignificant. The Zeeman Quantum beats due to the residual magnetic field may also affect lifetime measurements. No laboratory magnetic field other than the Earth's magnetic field was present at the excitation zone. However, no obvious signature of either the beats or distortions in the exponential decay curve was observed in the present and our previous works [9,12]. In the present work, lifetimes in the range 12–340 ns have been determined. For levels having lifetimes longer than 80 ns, a least-squares exponential fitting procedure to the fluorescence decay curve was used to obtain the lifetimes. For short-lived excited states, lifetime was determined by fitting the observed fluorescence decay curve with a convolution of a Gaussian function, which nearly represented the excitation laser pulse, and an exponential
decay function. The FWHM parameter of the Gaussian function, required in the convolution fit, was determined by recording the temporal shape of the excitation laser pulse with the same photomultiplier tube. A typical fluorescence decay curve for the 28,079.09 cm 1 level is shown in Fig. 1 along with the measured excitation laser pulse and the convolution fit which gave a lifetime value of 30.1 ns. The reported lifetime of a level is an average of about 10 different measurements performed under different experimental conditions like different excitation and ablation laser intensities as well as delay times. Lifetimes were also recorded in more than one decay as well as excitation channels, if they are strong. The radiative lifetimes for 37 odd-parity levels of Nb I obtained in the present work, with estimated uncertainties are presented in Table 1. To the best of our knowledge, lifetimes for 33 levels are reported for the first time. As a test case, we have measured lifetimes for four levels reported by earlier workers. Our lifetime values are 4–6% longer than those reported by Duquette et al. [3]. To check the systematic error in the measurements, if any, we measured lifetime of these levels by employing two different PMTs (Hamamatsu R943-02 and R1333) at different experimental conditions and we could not find any systematic error.
4. Conclusion Time-resolved laser-induced fluorescence spectroscopy in the supersonic free-jet was used to measure the radiative lifetimes of niobium atom. New measurements on 33 odd-parity levels of 4d35s5p and 4d45p configurations in the energy range of 18,790 and 35,730 cm 1 are reported. The present lifetime data in combination with the branching fractions can be used to determine the transition probabilities of the spectral lines originating from these levels. We intend to measure the branching fractions of the spectral lines using Fourier transform spectrometer. References [1] C.H. Corliss, W.R. Bozman Experimental transition probabilities for spectral lines of seventy elements, U.S. National Bureau of Standards monograph no. 53 U.S. Government Printing Office Washington, DC 1962, 226-46. [2] Biemont E, Grevesse N. f-Values and abundances of the elements in the sun and stars. Phys Scr 1977;16:39–47. [3] Duquette DW, Lawler JE. Radiative lifetimes in Nb I. Phys Rev A 1982;26:330–4. [4] Duquette DW, Den Hartog EA, Lawler JE. Absolute transition probabilities in Nb I and Hf I and a solution to the problem of missing infrared branches. J Quant Spectrosc Radiat Transf 1986;35:281–301. [5] Kwiatkowski M, Zimmermann P, Biémont E, Grevesse N. New lifetime measurements for Nb I and Rh I and the solar photospheric abundances of Nb and Rh. Astron Astrophys 1982;112:337–40. [6] Rudolph J, Helbig V. Radiative lifetimes in Nb I and Mo I using a novel atomic beam source. Phys Lett A 1982;89:339–41. [7] Malcheva G, Nilsson H, Engström L, Lundberg H, Biémont É, Palmeri P, et al. Radiative parameters of Nb I excited states. Mon Not R Astron Soc 2011;412:1823–7. [8] Meng B. The flight-out-of-view effect in radiative lifetime measurements of excited atoms by the method of laser-induced fluorescence. J Quant Spectrosc Radiat Transf 1989;41:303–5. [9] Nakhate SG, Mukund S, Bhattacharyya S. Radiative lifetime measurements in neutral zirconium using time-resolved laser induced
S. Mukund et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 166 (2015) 68–73 fluorescence in supersonic free-jet. J Quant Spectrosc Radiat Transf 2010;111:394–8. [10] Kramida A, Ralchenko Yu, Reader J, NIST ASD Team. NIST 2014 Atomic spectra database version 5.2 〈http://physics.nist.gov/PhysRef Data/ASD/levels_form.html〉. [11] Meggers WF, Corliss CH, Scribner BF. Tables of spectral-line intensities, Part I – arranged by elements monograph no. 145 1975: 175–87 [12] Yarlagadda S, Mukund S, Nakhate SG. Radiative lifetime measurements in neutral lanthanum using time-resolved laser-induced fluorescence spectroscopy in supersonic free-jet. J Opt Soc Am B 2011;28:1928–33.
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[13] Kröger S, Er A, Öztürk IK, Başar G, Jarmola A, Ferber R, et al. Hyperfine structure measurements of neutral niobium with Fourier transform spectroscopy. Astron Astrophys 2010;516:A70. [14] Er A, Öztürk IK, Başar G, Kröger S, Jarmola A, Ferber R, et al. Hyperfine structure study of atomic niobium with enhanced sensitivity of Fourier transform spectroscopy. J Phys B: At Mol Opt Phys 2011;44:205001. [15] Hannaford P, Lowe RM. Determination of atomic lifetimes using laser induced fluorescence from sputtered metal vapor. Opt Eng 1983;22:532–44.
Contents lists available at ScienceDirect
Journal of Quantitative Spectroscopy & Radiative Transfer journal homepage: www.elsevier.com/locate/jqsrt
Radiative lifetimes of odd-parity levels in Nb I Sheo Mukund a,b, Soumen Bhattacharyya a, Suresh Yarlagadda b, S.G. Nakhate a,b,n a b
Atomic and Molecular Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India Homi Bhabha National Institute, Bhabha Atomic Research Centre, Mumbai 400085, India
a r t i c l e in f o
abstract
Article history: Received 26 May 2015 Received in revised form 24 July 2015 Accepted 25 July 2015 Available online 4 August 2015
Radiative lifetimes are reported for 37 odd-parity energy levels of neutral niobium (Nb I), out of which 33 have been measured for the first time. The levels belong to electronic configurations 4d35s5p and 4d45p between 18,790 and 35,730 cm 1. The time-resolved laser-induced fluorescence spectroscopy technique was employed. The Nb atoms were generated in a free-jet by laser vaporization of niobium metal. Lifetime values reported in this work fall in the range 12–340 ns and are accurate to 710%. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Atomic spectra Radiative lifetime Branching fraction Time-resolved laser-induced fluorescence
1. Introduction Accurate radiative lifetimes of the excited atomic/ionic energy levels are of immense interest. The radiative lifetimes combined with accurate branching fractions, provide one of the most reliable methods for determining the absolute transition probabilities of spectral lines. Corliss and Bozman [1] have reported transition probabilities of spectral lines of many refractory metals based on the absolute emission measurements on intense arcs. However, the use of this data for determination of the solar abundance raises some doubts on their accuracy [2]. Apart from the basic interest in the radiative lifetimes, the transition probability values are required for numerous applications. The determination of the solar and stellar abundances of the chemical elements requires the knowledge of accurate transition probabilities. Accurate values of transition probabilities of transition metal elements are required in fusion research for the diagnosis of impurities in fusion-oriented plasma. Refractory metals such as niobium are used in the walls of plasma-confinement n Corresponding author at: Atomic and Molecular Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India. Fax: þ 91 22 25505151. E-mail address: [email protected] (S.G. Nakhate).
http://dx.doi.org/10.1016/j.jqsrt.2015.07.019 0022-4073/& 2015 Elsevier Ltd. All rights reserved.
devices. Radiative lifetime values are useful for assessment of the accuracy of theoretical data in atoms with complex electronic structure like transition metals and rare earths. In addition, spectroscopy based elemental analytical methods require knowledge of the oscillator strength. For the above reasons, the knowledge of accurate lifetime data is of continuing interest in atomic physics. The radiative lifetimes of Nb I have been reported in the literature by only four papers. Niobium, being a refractory metal, is difficult to vaporize using conventional thermal means. Several methods, like hollow-cathode effusive atomic beam, atomic beam formed by resistive heating of novel double cage metal wire and by electron bombardment of metal, and recently laser-produced metal plasma were employed to generate niobium atoms in gas phase. Lawler [3] have used time-resolved laser-induced fluorescence (TRLIF) in atomic beam based on hollow-cathode sputtering and measured radiative lifetimes of 50 odd-parity energy levels in the range 19,900–28,000 cm 1. The same group [4] have measured branching fractions of the Nb I spectral lines emerging from these odd-parity energy levels by using a hollow-cathode discharge source coupled to Fourier transform spectrometer and determined the absolute transition probabilities in 320 spectral lines by combining the radiative lifetime data determined earlier [3]. Kwiatkowski et al. [5]
S. Mukund et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 166 (2015) 68–73
2. Experiment The lifetime measurements in Nb I have been carried out in a free-jet apparatus. For details of the experiment readers are referred to read reference [9]. In brief, a rotating and translating niobium metal rod of 99.9% purity was ablated with a third harmonic beam of a Nd:YAG laser with a 10– 15 mJ/pulse energy focused to an 1 mm2 spot. The generated laser produced plasma was cooled and recombined by a supersonic helium gas pulse emanating from a nozzle of a pulse valve at about 210 kPa pressure, into a small channel and expanded freely into a vacuum chamber. Translationally cooled Nb atoms were excited to an energy level of interest, orthogonal to the supersonic expansion axis about 50 mm downstream the nozzle by a pulse dye laser having a pulse duration and bandwidth of 11 ns and 0.1 cm 1, respectively. A time synchronization and required delays between the gas pulse, ablation and excitation laser was achieved by a digital delay generator. Laser-induced fluorescence (LIF) was collected by a biconvex 50 mm f/1 lens along the axis orthogonal to both the atomic and laser beams and imaged on the entrance slit of the monochromator with magnification of 4 and detected by a cooled photomultiplier tube (PMT) having a risetime of 3 ns. The monochromator was used as a broad
band filter by keeping the entrance and exit slits 2.25 mm wide. The fluorescence decay signal detected by the PMT was further amplified by a fast amplifier (1 GHz bandwidth) and stored on a 200 MHz digital storage oscilloscope, triggered by the excitation laser pulse detected by a fast photodiode. The decay signal was averaged for 128 shots in order to obtain a good signal-to-noise ratio before transferring to a personal computer for analysis. 3. Measurements and results We have observed excitation lines of Nb I of reasonably high intensity originating from the ground and the low lying metastable levels up to the energy of 11,524.65 cm 1 [10]. The population of the Nb atoms in these metastable states after supersonic expansion was due to excitation processes in the laser-induced plasma. In view of the rich spectra of niobium atom, care was taken to identify correctly the transitions associated with the excited level of interest. In order to make sure that the correct level was excited; wavenumber calibration of excitation spectra with the accuracy of 0.1 cm 1 was carried out by using Ne optogalvanic spectral lines as well as commercial wavelength meter. In addition to this, the dispersed fluorescence (DF) spectrum was also recorded to confirm the fluorescence decay channels. The excitation line associated with the energy level of interest thus can be identified with high degree of confidence by comparing excitation wavelength and DF lines with spectral line list of Meggers et al. [11]. A few observed excitation and dispersed fluorescence lines not listed in [11], have also been classified by comparing the possible transition wavenumbers computed by using electric dipole selection rules of transitions among listed energy levels of Nb I [10] with observed wavenumbers. Fluorescence decay signals for lifetime determination of a level under investigation were usually recorded in more than one suitable decay channels. In addition to the statistical error, the measured lifetime gets modified due to contribution from the various effects which amounts to the systematic error. The effect of various experimental 0.5
Fluorescence Intensity (arb. units)
employed electron bombardment to generate Nb atomic beam and used TRLIF to measure radiative lifetimes in 11 odd-parity levels of Nb I. They deduced f-values from the measured lifetime data and branching fractions, calculated from Corliss and Bozman [1], and redetermined the photospheric abundance of niobium. Rudolph and Helbig [6] reported a new atomic source suitable for producing beams of refractory elements and measured lifetimes in six levels of niobium atom. Recently, Malcheva et al. [7] reported radiative lifetimes of 17 odd levels in Nb I in the energy range 27,400–47,700 cm 1 using TRLIF method in laser-produced plasma of niobium metal and compared with the theoretical calculations performed using a relativistic Hartree-Fock method including core polarization. By combining the experimental lifetimes and the calculated branching fractions, they obtain transition probabilities for the spectral lines emerging from the investigated levels. From so far published results, it appears that the lifetimes for most of the measured levels are consistent with each other. In spite of these efforts, radiative lifetime data for a number of oddparity levels are still missing. In this paper, we report radiative lifetime measurements on 37 levels of Nb I, out of which 33 are reported for the first time, in the region 18,790–35,730 cm 1 using time-resolved laser-induced fluorescence method in a supersonic free-jet. We have used the technique of laser vaporization in conjunction with supersonic expansion for generating atomic beam of a refractory element, niobium. The supersonic atomic beam allows the excitation of atoms in collision-free environment at low atom number density and thus measured radiative lifetimes are free from collisional and radiation trapping effects. However, because of the high translational velocity associated with the free-jet, this method suffers from the disadvantage of shortening of lifetime due to the flight-outof view effect [8], generally for longer lifetimes of the order of 1 ms or more.
69
0.4
0.3
0.2
0.1
0.0 0
50
100
150
200
250
Time (ns) Fig. 1. A typical fluorescence decay of the 28,079.09 cm 1 level of Nb I (circles). Triangles show recorded excitation laser pulse and the solid curve represents the convolution fit of the laser pulse with an exponential decay function that gave a lifetime value of 30.17 2.7 ns.
70
Table 1 Radiative lifetimes of 37 odd-parity levels of Nb I. Upper levela
Lower levela
Config.
Term
4d35s(5F)5p
Z 6F 0
4d4(a3P)5p
Z 4F 0
Z 2D 0
E (cm
)
J
E (cm
1
)
J
1/2 3/2 5/2 5/2
0.0 1142.79 1586.90 1586.90
532.022c 566.471 572.919 560.352
23,574.14 24,506.53
5/2 3/2 3/2 7/2
391.99 154.19 154.19 695.25
3/2
23,525.80
1/2
5/2
24,773.03
1/2
18,791.09
3/2 5/2
19,036.55 19,427.90
3/2
23,243.87
5/2 9/2
1
)
Lifetime (ns) This work
Previous work
1142.79
566.5
248.0(150)
3/2 7/2
1142.79 2154.11
558.7 578.7
289.5(195) 340.6(221)
437.478 432.973 426.867 419.851
3/2
1142.79
452.3
46.3(14)
44.4d
5/2 7/2 9/2
1586.90 2154.11 2805.36
454.7 447.2 460.7
47.3(6) 37.7(7)
44.5d, 42.7(20)e 35.9d, 33.2(17)e
0.0
424.946
695.25
415.204
4998.17 5297.92 5965.45 154.19 391.99 5965.45
539.6 548.5c 569.3 406.1 410.0 531.5
155.2(27)
7/2
1/2 3/2 5/2 3/2 5/2 5/2 3/2 1/2
5297.92 8410.90
537.1c 645.0c
71.5(31)
7/2
695.25
388.3
38.0(10)
5/2 7/2 5/2
1586.90 2154.11 5965.45
387.9 396.6 467.3c
40.7(47)
365.598 364.985 356.362 353.366
3/2 5/2 5/2
5297.92 5965.45 5965.45
450.3 458.2 444.7
22.9(11) 31.1(15) 16.0(10)
Z 2S 0
1/2
23,910.90
3/2
154.19
420.816
4d4(5D)5p
y 4F 0
9/2
26,440.33
9/2
1050.26
393.744
4
E (cm
λfluo (nm)b
3/2
4d4(a3P)5p
4 5
Lower level in fluorescence
4d ( D)5p
y D
0
5/2
27,359.70
3/2
4d35s(c3P)5p
y 4p 0
1/2 3/2 5/2
27,498.94 27,782.57 28,445.33
3/2 5/2 5/2 3/2
–
276o
5/2
27,614.10
3/2
154.19
364.064
7/2 5/2 7/2
695.25 1586.90 2154.11
371.4 384.1c 392.7
101.0(50)
4d35s(c3P)5p
x 4D 0
1/2
27,666.46
3/2
154.19
363.371
28,079.09
5/2
28,549.42
3/2 1/2 5/2
154.19 0.0 391.99
358.006c 356.040c 355.045
7/2
29,209.42
5/2 5/2
391.99 5965.45
346.918c 430.099
0.0 1142.79 5297.92 391.99 5297.92 695.25 5297.92 5965.45 695.25 2805.36 5965.45
361.3c 376.9 446.9 361.1c 438.8 358.9 430.0 442.7 350.6c 378.6 430.1
22.4(21)
3/2
1/2 3/2 3/2 5/2 3/2 7/2 3/2 5/2 7/2 9/2 5/2
154.19
154.19 391.99 391.99 154.19
367.472
c
64.6(8)
30.1(27) 15.7(7)
30.7(17)
4d35s(a3F)5p
z 2F 0
7/2
28,535.36
7/2
695.25
359.096c
7/2 5/2
2154.11 5965.45
378.9 442.9
123.9(72)
4d35s(a3F)5p
y 2G 0
9/2
28,433.74
9/2
1050.26
365.081
7/2
2154.11
380.4c
105.7(74)
35.6d
S. Mukund et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 166 (2015) 68–73
4d35s(a3F)5p
J
1
λexc. (nm)b
2
–
z p
0
3/2
27,918.85
1/2
0.0
358.083
c
c
354.402
5/2 1/2 5/2
1586.90 4998.17 5965.45
375.5 430.7c 449.4
12.3(6)
7/2 5/2 9/2
2154.11 5965.45 2805.36
362.1 420.1 366.0
45.5(17)
7/2
29,762.70
7/2
2154.11
362.103
9/2
30,117.32
9/2
1050.26
343.938c
1142.79
c
4d4(a1D)5p
x 4F 0
y 2F 0
4d4(3D)5p
v 4F 0
4d35s(1G)5p
x 2G 0
4d4(a1G)5p
w 2F 0
4d4(3H)5p a
4 0
I
3/2
29,622.73
3/2
351.029
c
38.1(14) c
1/2 3/2 3/2
4998.17 5297.92 5297.92
406.0 411.0 408.4
57.8(25)
3/2 5/2 7/2 5/2 5/2 9/2
1142.79 391.99 695.25 1586.90 1586.90 2805.36
349.1 337.8c 341.3c 352.0c 349.9c 365.4c
41.7(23) 34.4(11)
3/2 7/2 3/2 5/2 7/2
1142.79 2154.11 5297.92 1586.90 2154.11
338.0 350.0c 393.3c 339.6 346.3
47.0(17)
5/2
29,775.80
5/2
1586.90
354.653
3/2 5/2
29,779.44 29,987.45
5/2 3/2
1586.90 1142.79
354.607c 346.586
7/2
30,161.56
7/2
2154.11
356.947
5/2
30,716.50
5/2
1586.90
343.200c
7/2
31,025.52
5/2
1586.90
339.593
7/2
31,973.24
9/2
2805.36
342.745
9/2 5/2 5/2
2805.36 5965.45 9043.14
342.7 384.4 436.0
33.3(7)
32,213.94
9/2 9/2
2805.36 9328.88
339.940 436.843
9/2
2805.36
339.9
46.2(27)
7/2
32,087.58
9/2
2805.36
341.407
5/2 9/2 5/2
1586.90 12,102.12 13,404.77
327.8 500.2 535.1c
22.9(12)
15/2
35,731.34
13/2
11,524.65
412.993f
13/2
11,524.65
413.0
18.7(11)
9/2
Kramida et al. [10]. Wavelengths in air from Meggers et al. [11] except those marked with superscript “c.” c Transition wavelengths in air, not listed in Meggers et al. [11]. These are classified in this work and confirmed by dispersed fluorescence. d Duquette et al. [3]. e Kwiatkowski et al. [5]. f Classification by Kröger et al. Eur Phys J D 2007;41:61–70. b
66.1(44)
98.0(90)
32.4(14)
70.3(20)
S. Mukund et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 166 (2015) 68–73
y 4G 0
4d4(b3F)5p
29.3(18)
0.0
4d4(3H)5p
y D
436.2c 455.4 366.2c 426.4c 431.9c
351.495
1/2
4d 5s(a F)5p
4998.17 5965.45 1142.79 4998.17 5297.92
0.0
28,208.48
0
1/2 5/2 3/2 1/2 3/2
1/2
3/2
2
390.1c
28,442.16
z 4S 0
3
2805.36
1/2
4d4(a3P)5p
3
9/2
71
72
S. Mukund et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 166 (2015) 68–73
parameters on the systematic variation of the measured lifetime of a level was studied. Care was taken to minimize the influence on the lifetime measurements due to various effects like flight-out-of-view, radiation trapping, saturation, and collisional depopulation. The flight-out-of-view effect is a major spoiler, especially for longer lifetimes; in a free-jet setup and it was investigated rigorously in the present and our previous works [9,12]. The contribution from this effect was investigated by recording long lifetimes of the order of 300–400 ns at different delay time between the ablation and excitation lasers and also at lower backing pressures up to 140 kPa. Longer delay times probe the slower atoms than those excited at short delay time. Low backing pressure reduces the average atomic beam velocity. No obvious systematic variation in the lifetimes was observed with the variation in the delay and backing pressure, at least to the longest lifetime reported in the present work. The contributions of saturation and radiation trapping effects to the measured lifetimes were also investigated by measuring the lifetimes under different excitation laser pulse energies and number densities of Nb atoms. The Nb atom number density was varied by changing the ablation laser intensity. The lifetime values determined for a level with the variation in excitation and ablation laser intensity were also scattered randomly around a mean value. The error in the measurement due to nonlinear response of the photomultiplier tube at high fluorescence intensities was minimized by keeping the fluorescence intensity low either by inserting neutral density filters in front of the entrance slit of the monochromator or by exciting a transition with a low laser pulse energy. Since the present work was carried out in the supersonic free-jet operating at low base pressure o10 7 mbar, the lifetime reduction due to collisional depopulation was insignificant. The time resolved decay curve may exhibit a modulation at the hyperfine splitting frequencies. Niobium has only one naturally abundant isotope, 93Nb and it has the largest nuclear magnetic dipole moment among the stable nuclei. Thus the atomic levels of Nb have a broad hyperfine splitting [13,14]. As the frequency spread (1/Δτ 100 MHz) corresponding to the duration of the excitation pulse cannot cover the large hyperfine splitting, which is of the order of few hundreds of MHz, pairs of nondegenerate magnetic sublevels cannot be coherently excited [15]. Thus the systematic error due to modulations in the fluorescence decay signal due to hyperfine effect is insignificant. The Zeeman Quantum beats due to the residual magnetic field may also affect lifetime measurements. No laboratory magnetic field other than the Earth's magnetic field was present at the excitation zone. However, no obvious signature of either the beats or distortions in the exponential decay curve was observed in the present and our previous works [9,12]. In the present work, lifetimes in the range 12–340 ns have been determined. For levels having lifetimes longer than 80 ns, a least-squares exponential fitting procedure to the fluorescence decay curve was used to obtain the lifetimes. For short-lived excited states, lifetime was determined by fitting the observed fluorescence decay curve with a convolution of a Gaussian function, which nearly represented the excitation laser pulse, and an exponential
decay function. The FWHM parameter of the Gaussian function, required in the convolution fit, was determined by recording the temporal shape of the excitation laser pulse with the same photomultiplier tube. A typical fluorescence decay curve for the 28,079.09 cm 1 level is shown in Fig. 1 along with the measured excitation laser pulse and the convolution fit which gave a lifetime value of 30.1 ns. The reported lifetime of a level is an average of about 10 different measurements performed under different experimental conditions like different excitation and ablation laser intensities as well as delay times. Lifetimes were also recorded in more than one decay as well as excitation channels, if they are strong. The radiative lifetimes for 37 odd-parity levels of Nb I obtained in the present work, with estimated uncertainties are presented in Table 1. To the best of our knowledge, lifetimes for 33 levels are reported for the first time. As a test case, we have measured lifetimes for four levels reported by earlier workers. Our lifetime values are 4–6% longer than those reported by Duquette et al. [3]. To check the systematic error in the measurements, if any, we measured lifetime of these levels by employing two different PMTs (Hamamatsu R943-02 and R1333) at different experimental conditions and we could not find any systematic error.
4. Conclusion Time-resolved laser-induced fluorescence spectroscopy in the supersonic free-jet was used to measure the radiative lifetimes of niobium atom. New measurements on 33 odd-parity levels of 4d35s5p and 4d45p configurations in the energy range of 18,790 and 35,730 cm 1 are reported. The present lifetime data in combination with the branching fractions can be used to determine the transition probabilities of the spectral lines originating from these levels. We intend to measure the branching fractions of the spectral lines using Fourier transform spectrometer. References [1] C.H. Corliss, W.R. Bozman Experimental transition probabilities for spectral lines of seventy elements, U.S. National Bureau of Standards monograph no. 53 U.S. Government Printing Office Washington, DC 1962, 226-46. [2] Biemont E, Grevesse N. f-Values and abundances of the elements in the sun and stars. Phys Scr 1977;16:39–47. [3] Duquette DW, Lawler JE. Radiative lifetimes in Nb I. Phys Rev A 1982;26:330–4. [4] Duquette DW, Den Hartog EA, Lawler JE. Absolute transition probabilities in Nb I and Hf I and a solution to the problem of missing infrared branches. J Quant Spectrosc Radiat Transf 1986;35:281–301. [5] Kwiatkowski M, Zimmermann P, Biémont E, Grevesse N. New lifetime measurements for Nb I and Rh I and the solar photospheric abundances of Nb and Rh. Astron Astrophys 1982;112:337–40. [6] Rudolph J, Helbig V. Radiative lifetimes in Nb I and Mo I using a novel atomic beam source. Phys Lett A 1982;89:339–41. [7] Malcheva G, Nilsson H, Engström L, Lundberg H, Biémont É, Palmeri P, et al. Radiative parameters of Nb I excited states. Mon Not R Astron Soc 2011;412:1823–7. [8] Meng B. The flight-out-of-view effect in radiative lifetime measurements of excited atoms by the method of laser-induced fluorescence. J Quant Spectrosc Radiat Transf 1989;41:303–5. [9] Nakhate SG, Mukund S, Bhattacharyya S. Radiative lifetime measurements in neutral zirconium using time-resolved laser induced
S. Mukund et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 166 (2015) 68–73 fluorescence in supersonic free-jet. J Quant Spectrosc Radiat Transf 2010;111:394–8. [10] Kramida A, Ralchenko Yu, Reader J, NIST ASD Team. NIST 2014 Atomic spectra database version 5.2 〈http://physics.nist.gov/PhysRef Data/ASD/levels_form.html〉. [11] Meggers WF, Corliss CH, Scribner BF. Tables of spectral-line intensities, Part I – arranged by elements monograph no. 145 1975: 175–87 [12] Yarlagadda S, Mukund S, Nakhate SG. Radiative lifetime measurements in neutral lanthanum using time-resolved laser-induced fluorescence spectroscopy in supersonic free-jet. J Opt Soc Am B 2011;28:1928–33.
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[13] Kröger S, Er A, Öztürk IK, Başar G, Jarmola A, Ferber R, et al. Hyperfine structure measurements of neutral niobium with Fourier transform spectroscopy. Astron Astrophys 2010;516:A70. [14] Er A, Öztürk IK, Başar G, Kröger S, Jarmola A, Ferber R, et al. Hyperfine structure study of atomic niobium with enhanced sensitivity of Fourier transform spectroscopy. J Phys B: At Mol Opt Phys 2011;44:205001. [15] Hannaford P, Lowe RM. Determination of atomic lifetimes using laser induced fluorescence from sputtered metal vapor. Opt Eng 1983;22:532–44.