J. Phys. B: At. Mol. Opt. Phys.32 (1999) 4361–4371. Printed in the UK PII: S0953-4075(99)01903-3

Three-colour four-photon resonant excitation of theeven-parity autoionizing resonances in YbI

Raheel Ali†, Ali Nadeem†, M Yaseen†, M Aslam†, S A Bhatti‡ andM A Baig†† Atomic and Molecular Physics Laboratory, Quaid-i-Azam University, Islamabad, Pakistan‡ Applied Physics Division, PINSTECH, PO Nilore, Islamabad, Pakistan

Received 17 February 1999, in final form 22 June 1999

Abstract. We report new data on the even-parity autoionizing resonances due to the 4f inner-shell excitation in ytterbium using a three-colour four-photon resonant excitation technique. Thespectra were observed using three dye lasers simultaneously pumped by a common Nd:YAG laserin conjunction with a thermionic diode ion detector. More than 50 new levels corresponding to the4f136s2n` Rydberg series have been detected. The data were analysed in the light of the spectrallineshapes, parent ion configurations and quantum defect approach.

1. Introduction

Prior to the advent of tunable dye laser systems spectroscopic investigations were mainly carriedout via single-photon excitation using conventional light sources such as discharge lamps, flashtubes and synchrotron radiation. Dye lasers have made it possible to study selective even-or odd-parity levels by multi-photon excitation processes (Letokhov 1987, Demtroder 1998,Gallagher 1994). Ytterbium possesses a filled f subshell in addition to a filled s subshell in theground state, which is similar to that of the alkaline earths. However, due to the near energydegeneracy of the 4f and 5d orbitals, its spectra provide interesting phenomena of overlappinginner-shell excitation and double excitation, in contrast to the alkaline earth elements, wherethe spectra are dominated by the doubly excited levels above the first ionization threshold.Parr and Elder (1968) pioneered the study of the absorption spectra above the first ionizationthreshold using the hydrogen continuum as a background source generated by a DC discharge.They observed autoionizing lines at 193.2 and 188.0 nm, and a very broad line at 180.0 nmand also identified the 4f145dnp, 4f146pnd and 4f136s5dnd transitions. Subsequently, Kozlovet al (1976, 1977) and Kozlov and Kotochigova (1978) investigated the spectra using a 3 mspectrograph with a high-power hydrogen lamp and reported the autoionizing lines, whereas theanalysis was supplemented by the multi-channel quantum defect theory. Bekovet al (1980)reported the experimental and theoretical results of the doubly excited narrow autoionizingstates of 7s6p, 6p2, 5d6p, 5d2 configurations near the ionization limit using nitrogen-laser-pumped dye lasers. Baig and Connerade (1984), using the synchrotron radiation in conjunctionwith a 3 mspectrograph, reported the first observation of 4f13(2F7/2)6s2nd

[32

]1 andng

[32

]1

Rydberg series. The perturbations among the inner-shell and doubly excited channels wereanalysed with the help of multi-channel quantum defect theory.

Borisovet al (1986) reported four new autoionization resonances in the region 51 000–53 000 cm−1 using three-step laser spectroscopy and identified them with the help of the

0953-4075/99/174361+11$30.00 © 1999 IOP Publishing Ltd 4361

4362 R Ali et al

Hartree–Fock–Dirac calculations. Kaenderset al (1990) used the four-wave mixing techniqueto reinvestigate the autoionizing resonances in the wavelength region 141.5–139.1 nm.Griesmannet al (1992) measured the photoionization cross section and found that it isdominated by the strongly autoionizing resonances due to double excitations. The lifetimes,lineshape parameters and resonance energies of the broad overlapping resonance of the 5d6pconfiguration were extracted using Mies’ relations (Mies 1968).

Bi-ru et al (1992) studied the spectra of ytterbium using a Nd:YAG laser with a three-step excitation scheme and reported the autoionizing resonances in the energy range 50 443–57 743 cm−1. Xu et al (1994), using the isolated-core excitation scheme (Cookeet al 1978),studied the 6p(2P1/2)ns autoionizing resonances in the energy region 76 800–77 500 cm−1.Recently, Karapanagiotiet al (1995) reported the inner-shell excitation spectra of ytterbiumusing a multi-step excitation experiment. They populated the 6s8s1S0 level via the two-photontransition and the second laser was scanned above the ionization threshold revealing odd-parityautoionizing resonances which were compared with the spectra recorded by a single photonexcitation (Baig and Connerade 1984). More recently, Jong-hoon (1995) investigated 17 odd-parity autoionizing levels in the energy region 50 400–58 000 cm−1 and reported theJ -valuesand linewidths of these resonances. In their experiment the autoionizing states were excitedfrom the 4f13(2F7/2)6s2P3/2 J = 2 intermediate level, which was populated from the groundstate via the 6s6p3P1 resonance by a two-step resonant excitation.

In the present work we have studied the autoionizing spectra of ytterbium in the energyregion 70 000–72 000 cm−1 using a three-colour four-photon resonant excitation scheme. Theobserved data reveal 4f13(2F7/2)6s2n` narrow autoionizing Rydberg series withn values up to45.

2. Experimental details

We have used three Hanna-type (Hannaet al1975) dye lasers pumped by the second and thirdharmonics of a Nd:YAG laser to record the spectrum of ytterbium in the energy region 70 000–72 000 cm−1. The first dye laser, pumped with the second harmonic and charged with the R-610dye dissolved in ethanol, was tuned at a wavelength of 582.3 nm that excited the ytterbiumatoms from the ground state 4f146s2 1S0 to 4f146s7s1S0 via two-photon excitation. The seconddye laser, also pumped with the second harmonic and charged with the LDS-698 dye, was tunedat 682.3 nm to excite the ytterbium atoms from 4f146s7s1S0 to the 4f13(2F7/2)6s25d

[32

]1 level

at 49 005.74 cm−1 that was identified by Baig and Connerade (1984). The third laser, pumpedwith the third harmonic and charged with the C-460 and C-440 dyes, was scanned in thewavelength region from 475 to 438 nm to study the spectra above the first ionization limit. The4f13(2F7/2)6s25d

[32

]1 intermediate level was also populated via a second excitation path: the

first laser was adjusted to the 6s6p3P1 level and with the second laser a two-photon excitationpromoted the atoms to the 4f13(2F7/2)6s25d

[32

]1 level, whereas the third laser was scanned

to record the autoionizing resonances built on the 4f13(2F7/2)6s2 ionic level. By employingthese two different excitation paths only theJ = 1 component of the intermediate level wasselected. The schematic energy level diagram of the excitation paths and the relevant energylevels are shown in figure 1.

The linewidths of all the dye lasers were≈ 0.3 cm−1. The wavelength tuning was achievedby rotating the rotary stage of the dye laser grating by a computer-controlled stepper motor.All the laser beams were overlapped and were optimized for a maximum ion signal. Thewavelength was initially adjusted with an accuracy of±0.1 nm by a 1 m monochromatorequipped with a 1200 lines/mm holographic grating. The wavelength calibration was achieved

Even-parity autoionizing resonances in YbI 4363

Figure 1. Excitation scheme used to record the even-parity autoionizing resonances in neutralytterbium.

by injecting part of the dye laser beam into a neon hollow cathode lamp and recording theoptogalvanic spectra of neon (Harrison 1982). For the relative frequency markers, a smallamount of the dye laser light was passed through a solid fused silica etalon of 1 mm thickness.The term energies of the excited levels have been determined with an accuracy of±0.5 cm−1

for the sharp lines.A single-wire thermionic diode was used to detect the ions which has been used very

successfully in our laboratory to study the Rydberg spectra of lithium (Baiget al 1994),sodium (Baiget al 1995) and potassium (Akramet al 1997). It consists of a stainless steeltube of length 35 cm, inner diameter 3 cm and wall thickness 1 mm. The diode detector can beheated up to 1000◦C by a DC power supply. Both ends of the furnace tube were water cooledand sealed with 2.5 cm diameter quartz windows. The ultimate temperature was monitored byan Ni–Cr–Ni thermocouple and it can be maintained within±2% by a temperature controller.A molybdenum wire of length 20 cm and diameter 0.3 mm extending along the axis of thestainless steel tube was used as the collection electrode, which was heated by a separateregulated DC power supply capable of delivering a current of 4–5 A at 10 V.

The system was evacuated up to 10−5 mbar and heated for several hours. About 2 gof ytterbium sample with a small amount of potassium (to reduce the work function of the

4364 R Ali et al

filament) was then loaded into a stainless steel boat. Argon gas at a pressure of 1–1.5 mbarwas used as a buffer gas. The cell was heated to a temperature of 500± 5 ◦C correspondingto a vapour pressure of≈10−2 mbar of ytterbium. The cathode current was optimized bymonitoring the ion signal. The change in the diode current due to the photo-ion productionwas measured as the voltage drop across a load resistor (100 k�).

Three independent detection channels have been used to record the data. The signals fromthe thermionic diode ion detector, the hollow cathode lamp and the solid etalon were sampledsimultaneously by three independent boxcars (SR-250) and stored on a personal computerinterfaced through a GPIB card NI 488.

3. Results and discussion

The sub-valence electrons in atoms can be excited, after the valence-shell excitation, byutilizing higher-energy photons and, consequently, more and more excitation channels becomeavailable with further increasing energy. Sharp inner-shell transitions may occur when theelectrons are excited to atomic orbitals with larger`-values, because the excited states areheld out from the core by the centrifugal potential barrier (Cowan 1981, Baig and Connerade1984). The situation is very interesting in ytterbium because the structure due to the doubleexcitation, simultaneous excitation of both the 6s electrons, and the 4f inner-shell excitationoverlap in energy. However, the structure of the 4f inner-shell excitation remains sharp andweak, the 4f13(2F7/2)6s2 convergence limit 71 861.75 cm−1 lies about 21 418 cm−1 abovethe first ionization threshold, whereas the double-excitation threshold 5d(2D3/2,5/2) lies at74 775.69 cm−1. The Rydberg structure due to both these excitation channels interacts. Theinterchannel interactions among theJ = 1 channels were analysed by Kozlovet al (1976,1977), Kozlov and Kotochigova (1978), Baig and Connerade (1984) and Baiget al (1992)using the MQDT technique.

Figure 2 shows the ionization signal covering the scanning laser energy from 22 220 to22 405 cm−1. In order to assign the observed lines, it is important to look at the excitationprocesses. The ground state of ytterbium 6s2 1S0 is even parity and with the first step by atwo-photon excitation we approach the 6s7s1S0 (even parity). In the second step, the atomsare excited to 4f13(2F7/2)6s25d

[32

]J = 1 (odd parity). In the third step these atoms are excited

to 4f13(2F7/2)6s2n` (even-parity) Rydberg levels withJ = 0, 1 and 2 since the intermediatelevel possessesJ = 1. The excited levels are designated in thejcK-coupling scheme, in whichthe dominant interaction is the spin–orbit splitting of the 4f13(2F7/2,5/2) core levels. In thiscoupling scheme the total orbital angular momentum and the spin quantum number of the coreelectrons are coupled to form thejc quantum number. Thejc quantum number is then coupledto the orbital angular momentum of the excited electron to form theK quantum number.K isthen weakly coupled to the spin quantum number of the excited electron to give the resultantJ quantum number (Cowan and Andrew 1964). Therefore, the possible Rydberg series are

4f13(2F7/2)6s25d[

32

]1→ 4f13(2F7/2)6s2np

[52

]2

→ 4f13(2F7/2)6s2nf[

12

]0,1,

[32

]1,2 and

[52

]2.

Only onenp and threenf series are expected within the framework of thejK-couplingscheme, since thenp series follow 1K = 1J = 1` selection rules, and theassignment is straightforward. However, the observednf series is tentatively assigned as4f13(2F7/2)6s2nf

[52

]2 in the light of the dipole selection rules(1K = 1J = +1`). The

Rydberg levels are therefore identified as 4f13(2F7/2)6s218–19p[

52

]2 and 4f13(2F7/2)6s215–

16f[

52

]2, respectively. There are four broad resonances (marked with an asterisk) at

Even-parity autoionizing resonances in YbI 4365

Figure 2. Ionization signal in the laser-energy region between 22 220 and 22 405 cm−1, showingthe 4f13(2F7/2)6s218–19p[52 ]2 and 4f13(2F7/2)6s215–16f[52 ]2 autoionizing resonances. Theunidentified resonances or resources whose tentative assignments are given are marked by anasterisk (see table 3).

22 240.7 cm−1 (0 = 3.2 cm−1), 22 248.1 cm−1 (0 = 2.4 cm−1), 22 256.5 cm−1 (0 =11.8 cm−1) and 22 395.6 cm−1 (0 = 3.4 cm−1), respectively, in addition to a very strongline at 22 316.0 cm−1 (0 = 0.9 cm−1). These lines disappear by blocking any one ofthe fixed dye lasers, which confirms that they originate due to the combination of all ofthe lasers. We propose here only a tentative assignment of these resonances. The broadline around 22 256.5 cm−1 is assigned as doubly excited levels 6p(2P1/2)8p

[12

]1 and the

line at 22 395.6 cm−1 as 6p(2P1/2)8p[

32

]1. The sharp line at 22 316.0 cm−1 is assigned as

4f13(2F5/2)6s27p[

32

]1, the lower member of the series built on the 4f13(2F5/2)6s2 parent ion

level. The line at 22 380.2 cm−1 is identified as 6p2 3P2 as a direct two-photon excitation fromthe ground state. The 6p2 3P0 line is also observed at 21 218.5 cm−1, but not shown in thisfigure.

In figure 3, we reproduce the autoionizing resonances recorded in the laser-energy range22 405–22 635 cm−1, keeping the first two lasers fixed and scanning the third laser. Thereare two sequences of members of the Rydberg series in addition to two strong resonances at22 463.5 cm−1 (0 = 3.0 cm−1) and 22 596.0 cm−1 (0 = 2.2 cm−1), respectively. By lookingat the effective quantum numbers, the Rydberg levels are assigned withn = 20–26 andn = 17–23 of the 4f13(2F7/2)6s2np

[52

]2 and 4f13(2F7/2)6s2nf

[52

]2 series, respectively. In this figure

the first resonance 4f13(2F7/2)6s220p[

52

]2 is stronger than the nearby 4f13(2F7/2)6s217f

[52

]2

line. The intensity difference among the` = 1 and 3 levels can be seen more clearly for the4f13(2F7/2)6s224p

[52

]2 and 4f13(2F7/2)6s221f

[52

]2 levels—this intensity variation is persistent

throughout the spectra. It is interesting to note that the series attached to the 4f inner-shellexcitation are very weak and a similar behaviour was observed in the case of the single-photon4f13(2F7/2)6s2nd andng J = 1 excitation from the ground state (Baig and Connerade 1984,

4366 R Ali et al

Figure 3. Ionization signal in the energy region between 22 405 and 22 635 cm−1 showingthe higher members of the 4f13(2F7/2)6s2np[ 5

2 ]2 (20 6 n 6 26) and 4f13(2F7/2)6s2nf[ 52 ]2

(176 n 6 23) autoionizing resonances. The resonances whose tentative assignments are givenare marked by an asterisk (see table 3).

Baig et al 1992c). The lower transition probabilities of the Rydberg series stressed that theeffective radial potential of the f-electron contains a centrifugal term, as a result of whichthe bound f-states are segregated from the core. Therefore, most of the oscillator strength isconcentrated in a delayed onset effect (Connerade 1998) towards higher energies above theionization threshold.

The two dominating autoionizing resonances at 22 463.5 cm−1 (0 = 3.0 cm−1) and22 596.0 cm−1 (0 = 2.2 cm−1) are tentatively assigned as 4f13(2F5/2)6s27p

[32

]2 and

4f13(2F5/2)6s27p[

52

]2, respectively, pertaining to 4f inner-shell excited levels built on the

4f13(2F5/2)6s2 parent ion levels. However, definite level assignments will only be possibleafter extensiveab initio calculations that are not available at the moment.

Figure 4 shows the spectra in the energy region 22 635–22 810 cm−1. The presenceof the two Rydberg series is more evident here. The 4f13(2F7/2)6s2np

[52

]2 series observed

up to n = 45 remains stronger than the 4f13(2F7/2)6s2nf[

52

]2 series. We observed the

4f13(2F7/2)6s2nf[

52

]2 series up ton = 33 because the intensities of the higher members

reduce drastically. A sudden enhancement in intensities of the series around 22 725 cm−1 isworth noting that may be attributed to the presence of three additional lines at 22 714 cm−1

(0 = 1.4 cm−1), 22 722.5 cm−1 (0 = 1.8 cm−1) and 22 728.7 cm−1 (0 = 1.8 cm−1). The4f13(2F7/2)6s231f

[52

]2 line adjacent to these perturbing levels gains oscillator strength at the

expense of the 4f13(2F7/2)6s234p[

52

]2 line. We believe that these lines belong to the levels

built on the 4f13(2F5/2)6s2 parent ion level.

Even-parity autoionizing resonances in YbI 4367

Figure 4. Ionization signal in the laser-energy region between 22 635 and 22 810 cm−1 showing thehigher members of the 4f13(2F7/2)6s2np[ 5

2 ]2 and 4f13(2F7/2)6s2nf [ 52 ]2 autoionizing resonances.

Unidentified resonances are marked with an asterisk.

The observed data are arranged into Rydberg series using the Rydberg formula:

En = IP − Ry

(n− µ)2whereRy is the mass corrected Rydberg constant for ytterbium, 109 737.07 cm−1, IP is theseries limit andµ is the quantum defect, which remains constant for an unperturbed Rydbergseries and thus can be used for the identification of the series members.

The term energies and effective quantum numbers are tabulated in tables 1 and 2 forthe 4f13(2F7/2)6s2np

[52

]2 and 4f13(2F7/2)6s2nf

[52

]2 series, respectively. The unidentified

lines are collected in table 3. It is interesting to note that the widths of the lines are verysmall (0 ' 1–2 cm−1) in spite of the fact that these are the autoionizing resonances. Thewidths of the lines attached to doubly excited levels 6p(2P1/2)8p

[12

]1 at 22 256.5 cm−1 and

6p(2P1/2)8p[

32

]1 at 22 395.6 cm−1 are0 = 11.8 and 3.4 cm−1, respectively. Bekovet al

(1980) reported the widths of the 7s6p3P0,1,2 and 1P1 odd-parity autoionizing resonancesas0 = 1.2, 0.95, 1.6 and 2.6 cm−1, respectively. Bekovet al (1980) also calculated thewidths of the 5d6p (odd-parity) and 6p2, 5d2 (even-parity) configuration-based levels. Thewidths of all these autoionizing resonances remain very small, less than 1 cm−1, except forthe two resonances 6s6p1S0 (0 = 5.4 cm−1) and 5d6p1P1 (0 = 5.7 cm−1). Subsequently,Jong-hoonet al (1995) reported 17 odd-parity autoionizing resonances in the energy range50 400–58 000 cm−1 with J = 1, 2, 3 using the 4f13(2F7/2)6s26p3/2 J = 2 as an intermediatelevel. They established theJ -values for the upper levels using the polarization spectroscopictechnique. Some of the resonances were very broad, particularly theJ = 3 resonancesat 52 187 and 57 861 cm−1 (0 = 106 and 70.3 cm−1), and theJ = 2 resonance at57 213 cm−1 (0 = 85 cm−1). They also reported twoJ = 1 resonances at 51 520.06 cm−1

4368 R Ali et al

Table 1. Rydberg series resulting from the inner-shell excitation of the 4f subshell through three-colour four-photon excitation in ytterbium. 4f13(2F7/2)6s2np[ 5

2 ]2.

Term energy FWHMn (cm−1) n∗ 0 (cm−1)

12 70 120.5 7.94 1.513 70 482.0 8.92 0.414 70 754.2 9.95 1.315 70 914.8 10.77 1.216 71 091.2 11.93 0.417 71 207.4 12.95 1.218 71 295.4 13.92 1.419 71 368.1 14.91 1.920 71 428.1 15.91 0.621 71 478.3 16.92 0.822 71 519.2 17.90 0.623 71 554.4 18.90 0.824 71 584.2 19.89 0.725 71 612.5 20.98 1.326 71 633.3 21.92 0.627 71 649.4 22.74 0.528 71 670.2 23.93 0.629 71 685.3 24.94 0.530 71 698.8 25.95 0.531 71 710.3 26.92 0.432 71 721.1 27.94 0.633 71 731.7 29.05 0.634 71 740.2 30.04 0.235 71 747.1 30.94 0.336 71 753.5 31.84 0.337 71 759.6 32.77 2.138 71 766.0 33.74 2.039 71 770.7 34.72 2.140 71 775.7 35.71 2.241 71 780.3 36.71 1.842 71 783.6 37.5 0.643 71 788.0 38.6 1.444 71 791.2 39.4 0.345 71 794.6 40.4 0.6

(0 = 50.3 cm−1) and 51 805.19 cm−1 (0 = 35 cm−1). In the energy region 51 000–59 000 cm−1 Griesmannet al (1992) and Baiget al (1992c) reported five very broadJ = 1autoionizing resonances: 5d(2D3/2)6p

[12

]1 at 51 771 cm−1 (0 = 58 cm−1), 5d(2D3/2)6p

[32

]1

at 53 073 cm−1 (0 = 542 cm−1), 4f135d26s5D1 at 53 985 cm−1 (0 = 37 cm−1), 4f135d26s3S1 at 54 281 cm−1 (0 = 942 cm−1) and 5d(2D5/2)6p

[32

]1 at 54 552 cm−1 (0 = 2287 cm−1).

One of the twoJ = 1 resonances observed at 51 805.19 cm−1 by Jong-hoonet al (1995)may be correlated to the 5d(2D3/2)6p

[12

]1 level, whereas the assignment of the other level

needs further investigation. Borisovet al (1986) calculated the level structures in the energyregion 51 000–53 000 cm−1, including the 4f136s6p2, 4f136s5d2 and 4f136s26p configurationsusing the Hartree–Fock–Dirac method and reported61.5% agreement between the computedand experimental values. Unfortunately, Borisovet al (1986) did not include the 4f145d6pconfiguration in their calculations although the level structures based on this configuration

Even-parity autoionizing resonances in YbI 4369

Table 2. Rydberg series resulting from the 4f inner-shell excitation via three-colour four-photonprocess in ytterbium. 4f13(2F7/2)6s2nf[ 5

2 ]2.

Term energy FWHMn (cm−1) n∗ 0 (cm−1)

9 70 190.8 8.10 0.910 70 512.7 9.02 0.811 70 762.5 9.99 1.412 70 963.6 11.05 0.513 71 105.0 12.04 0.414 71 242.6 13.31 0.915 71 312.2 14.13 1.416 71 377.3 15.05 0.417 71 432.6 15.99 1.718 71 483.2 17.03 6.719 71 525.5 18.06 0.320 71 559.1 19.04 1.221 71 588.8 20.05 0.822 71 613.6 21.03 0.523 71 635.6 22.03 0.324 71 652.8 22.91 0.325 71 672.5 24.08 0.526 71 687.5 25.09 0.527 71 700.0 26.05 0.528 71 711.5 27.02 0.429 71 722.5 28.07 0.230 71 732.4 29.13 0.431 71 741.2 30.17 0.532 71 748.2 31.09 0.833 71 755.4 32.12 0.3

dominate in the autoionization region. The more complete calculations are those of Wyartand Camus (1979), who computed the level structures of 4f136s26p, 4f146p2, 4f135d26s and4f145d6p configurations. Karapanagiotiet al (1995) reported the odd-parity autoionizingresonances attached to the 4f inner-shell excitation via multi-step laser excitation fromthe 6s8s1S0 intermediate level and remarked that the widths of the lines are small andcomparable to the resonances observed in the synchrotron-radiation-based single-photonabsorption experiment. From the present study of the even-parity autoionizing resonances,we conclude that the 4f inner-shell excitation reveals very narrow autoionizing resonances.Broad autoionizing resonances correspond to the simultaneous excitation of both the 6svalence electrons attached to the 6p parent ion level, whereas relatively sharp resonancesare attached to the 4f13(2F5/2)6s2 parent ion levels of ytterbium. The broad nature of theautoionizing resonances corresponding to the double excitation can be attributed to the largerautoionizing probabilities of these resonances into the adjacent continuum. The sharpness ofthe autoionizing resonances due to the 4f inner-shell excitation is a signal of the presence of thedelayed onset effect (Cowan 1981, Connerade 1998)—all of the oscillator strengths of thesediscrete levels lie in the 4f continuum. It will be interesting to look for the delayed onset inthe 4f continuum using a broadband laser or a clean continuum source such as synchrotronradiation.

In conclusion, more than 50 even-parity narrow autoionizing resonances that correspondto the 4f13(2F7/2)np

[52

]2 andnf

[52

]2 Rydberg series of ytterbium resulting from 4f inner-shell

4370 R Ali et al

Table 3. List of the unidentified levels resulting from the three-colour four-photon excitation inytterbium.

Laser energy(cm−1) FWHM 0(cm−1)

21 263.3 1.321 440.2 1.821 517.0 1.721 526.8 3.021 669.2 1.222 240.7∗ 3.222 248.1∗ 2.422 256.5∗ 11.822 316.0∗ 0.922 395.6∗ 3.422 463.5 3.022 596.0 2.222 639.3 0.722 648.4 0.322 669.8 0.422 714.0 1.422 722.5 1.822 728.7 1.8

∗ A tentative assignment is given in the text.

excitation are reported. It is noticed that the even-parity autoionizing resonances remain asnarrow as the odd-parity autoionizing resonances regardless of the excitation mechanisms.

Acknowledgments

We thank the Pakistan Science Foundation (PSF-104), Alexander von Humboldt Foundation(Germany), Physikalisches Institut der Universitat Bonn (Germany), ICTP Trieste (Italy) underthe ICAC scheme, Quaid-i-Azam University, Islamabad (URF) and the PAEC (Pakistan) forproviding financial support. RA and MY are grateful to the ICAC for financial support.

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Even-parity autoionizing resonances in YbI 4371

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