Some lifetimes and transition probabilities in Cu(I)

J. Quant. Spectrosc. Radiat. Transfer Vol. 28, No. 5, pp. 383-387, 1982

0022~073[82]110383-05503.0010

Printed in Great Britain.

© 1982Pergamon Press Ltd,

SOME LIFETIMES AND TRANSITION PROBABILITIES IN Cu(I) AKIHIROKONOand Sr~uzo HATrORI Department of Electronics,NagoyaUniversity,Chikusa-ku,Nagoya464, Japan

(Received 29 January 1982) Abstract--Radiativelifetimesof 7 levels in the Cu(l) 3d9 4s4p configurationhave been measuredusing a delayed coincidence technique; copper vapor was excited in an argon buffer gas and cross sections for collisional destruction of copper levels by argon were also obtained. Transition probabilitiesof the lines originating from these levelshave been determinedusing the measuredlifetimesand branchingratios. INTRODUCTION Although a number of transition-probability data are available for Cu(I), ~'2 few of these are directly based on lifetime measurements. We have measured the lifetimes of 7 levels in the CU(I) 3d 9 4s4p configuration using a delayed coincidence technique. Since copper vapor was excited in an argon buffer gas, cross sections for collisional destruction of copper levels by argon were also obtained. To determine transition probabilities, we have also measured the branching ratios, except for a few weak lines. LIFETIME MEASUREMENTS The apparatus is the same as that used previously for Cu(II) measurements 3 and is only briefly described here. Production and pulsed excitation of copper vapor were accomplished in a specially designed discharge tube. It consists of a planar meshed anode, a cylindrical copper hollow cathode placed on one side of the anode, and an accelerating electrode placed on the other side of the anode. A d.c. discharge is maintained between the cathode and the anode. Copper vapor is produced by cathodic sputtering and diffuses through the anode mesh. A pulsed voltage is applied between the accelerating electrode and the anode to excite the vapor by impact of accelerated electrons in the discharge plasma. The optical decay of an excited level is recorded with a 256-channel delayed coincidence analyzer. The fall time of the pulsed voltage used was 2 ns. For high-lying levels (e.g. E > 15 eV), it has been demonstrated in the Cu(II) measurements 3 that turning off of excitation is sufficiently rapid for lifetimes as short as 2 ns to be measured. For low-lying levels, however, turning off of excitation is not rapid enough. We measured the decay of the Cu(I) 4p 2p3/2 level (E = 3.8 eV) and found that the decay rate corresponds to a lifetime of about 20ns. This value is considerably larger than the reported lifetimes of about 7 ns. *'~° This result indicates that the observed decay rate reflects a decreasing rate of the population of the electrons which are capable of exciting this level. To ensure that radiative decay rate is observed, we restricted measurements to levels whose lifetimes are much longer than 20 ns. Measurements were made for the levels 4s4p(3p)2Ds[2, 2F512, 2F712, 4p1/2, 4p3/2, 4D312, and 4Dm in the Cu(I) 3d 9 4s4p configuration; the lines used were at 2824, 3280, 2961, 2442, 2492, 3194, and 3011 ]k, respectively.t These levels decay to the metastable levels 3d 9 4s 2 2D, while those with J = 1/2, 3/2 decay also to the ground state 4s 2S. Since all these transitions are forbidden in the strict LS-coupling scheme, these levels have relatively long lifetimes. Average channel widths used for the present measurements were 10.8 ns for the 2Dst 2 level and 17.9 ns for the other levels. Decay curves were fitted to a sum of exponential terms (or a single exponential term) plus a background constant using a computer program for nonlinear least-squares analysis. In the curve-fitting process, data in those channels were excluded which correspond to a time interval of several tens of nanoseconds after cutoff of the excitation pulse. Any effect of cascading should thus be eliminated from the results since major cascading levels for the levels under study are expected to have short lifetimes. This hypothesis was cor+In this paper, leveldesignationsfor the 3d9 4s4p levelsfollow Landman,J. Opt. Soc. Am. 59, 962 (1%9). 383

384

A. KONOand S. HATTOR[

roborated by the fact that the observed decay curves were well fitted to a single exponential term plus a background constant, except for the 4D3/2 and 4D~/2 levels. For these two levels, the decay curves were resolved into two exponential components; the decay rate of the component with the larger amplitude was adopted as the decay rate for the level under consideration. The origin of double-exponential decay for these levels is discussed in the last section, The discharge was sustained with an argon buffer gas. On the basis of spectral intensity measurements, it is deduced that the copper atom density is roughly proportional to the discharge current at a fixed argon pressure. For the 4P1/2 and 4p3/2 levels, dependence of the decay rates on the discharge current was studied for currents ranging from 5 to 25 mA at an argon pressure of 0.25 tort, but no significant dependence was detected. These results indicate that the effect of resonance trapping is negligible in the present measurements. The dependence of the decay rates on the argon pressure was studied for pressures ranging from 0,25 to 0.75 tort at a discharge current of 10 mA; the results are given in the last section. BRANCHING-RATIO MEASUREMENTS

To determine transition probabilities, intensity ratios of lines sharing the same upper levels were measured photoelectrically, except for a few weak lines. The light source used was ~:1 copper hollow-cathode lamp operated at a discharge current of 10 mA with 1 torr argon used as a buffer gas. The relative spectral sensitivity of our optical system in the spectral range from 2000 to 3500 X, was determined by multiplying the following quantities: relative sensitivity of the photomultiplier, relative efficiency of the monochromator, and relative transmittivity of the quartz window in the hollow cathode lamp. The efficiency of the monochromator and the transmittivity of the quartz window were measured by using a deuterium lamp and another monochromator as a monochromatic light source. The sensitivity of the photomultiplier was calibrated by the photomultiplier supplier (Hamamatsu). The relative sensitivity of the entire system above 3000A was checked by measurements with a standard lamp. The sensitivity determined by the two methods agreed within the experimental uncertainties. RESULTS

The dependence of decay rates on argon pressure is shown in Fig. 1. For the levels 2Ds/2, and 4P~/2, the effect of collisional destruction by argon is not significant. The lifetimes of these levels are therefore determined as the means of several values obtained at an argon pressure of 0.25 torr; these mean values are plotted in Fig. I for the decay rates at this

2F5/2, 4Pl/2,

-~ ....

o - - - --o- . . . . . .3-- ---Q- 2 D 5 / 2

3 -__~ .....

5,.,', 4

%

"63

~o- . . . . .

2

5 ....

~

o-- 4 P 3 / 2

2F7/2

~

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g 2

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.....

~o..____~ --~--

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0%

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o16

Ar p r e s s u r e (Torr) Fig. I. Dependence of the decay rates of Cu(l) levels on argon pressure.

Somelifetimesand transitionprobabilitiesin Cu(I)

385

pressure. For the levels 2F7/2, 4D3/2, and 4D5/2, the pressure dependence of the decay rates is apparent. The lifetimes of these levels are determined by fitting decay rates to the relation r

=

a + nvq,

(1)

where r is the decay rate, n is the density of argon atoms, v is the mean relative velocity between copper and argon atoms, and a and q are adjustable parameters corresponding, respectively, to the radiative decay rate and the cross section for collisional destruction of copper levels by argon. The cross sections obtained are given in Table 1. It is noteworthy that the observed decay curves for the 4D3/2 and 4D5/2 levels have such features as to suggest that collisional destruction of these levels is due to the reciprocal process Cu(4D3/2) + Ar ~ Cu(4Ds/2) + Ar + 17 meV.

(2)

Specifically, these features are (1) The decay curves of these levels consist of two exponential components; (2) The decay rates of the two components for one level are identical, within the experimental uncertainties, with those for the other level obtained in the same experimental conditons; (3) The faster-decaying components for the longer-lived level (4D5/2) have negative coefficients. The lifetimes and the transition probabilities are given in Table 2, together with the results of other workers. The errors in the measured lifetimes given in the table include 1% uncertainties in the calibration of the average channel widths of the delayed coincidence analyzer and twice the statistical standard deviations; the standard deviations for the aFvl2, 4D3/2, and 4D~a levels are given by a least-squares fit of decay rates to the function given by Eq. (1), and those for the other levels are simply the rms deviations of several values from the mean values. For the 4P3/2 and 4D3n levels, the results of other lifetime measurements are available. Bucka et al. '~ and Krellmann et al. 8 used a level-crossing technique, and Kowalski and Putlitz n used an optical double resonance technique. Differences between our results and those of others are less than 10% and fall within experimental uncertainties. The transition probabilities in this work are determined by using the measured lifetimes and branching ratios, neglecting the weak lines for which intensity measurements were not made. The results of other workers in Table 2 indicate that the errors caused by the neglect of these lines are at most a few percent. The results in this work are believed to be accurate to about 10%, except for the 2244, 3194, and 3458-~k lines, for which 30% would be a safe estimate; the transition probabilities for these three lines are sensitive to the errors in the spectral sensitivity calibrations of the optical system since they were determined by using relative intensities of well separated lines. A number of comparisons with other experiments are possible. Kock and Richter ~3 made emission measurements under LTE conditions. Corliss ~ renormalized the results of similar measurements by Meggers et al., t4 Corliss, t5 and Allen and Asaad 16 to an improved scale by using the results of Kock and Richter.t Slavenas ~7 used the hook method. Ostroumenko and Rossikhin, TM and Lvov t9 made absorption measurements. The relative values reported in Refs. tA similarrenormalizationwas madeby Bielski(Ref.2) to a differentscale.Differencebetweenhis resultsand those of Corliss are smallfor the linesin Table2. Table 1. Cross sectionsfor collisionaldestructionof Cu(I)levelsby argon. Cross section a (i0-16cm2 )

Level 4s4p(3p)2F7/2

Ii,i+I.i

4D3/2

2,8+1.9

4D5/2

4,3+0.8

a. A gas temperature of 300 K is assumed. The quoted errors are twice the standard deviations given by the fitting process.

A. KONOand S. H~r-lorl

386

Table 2. Lifetimes and transition probabilities in Cud). 7. . . . . . .

T /

Upp,~ .

. . . . . . . . ! Litetime ( s} i "ibis

3 ,2

4s4p(P)

'7~Other

.•

:

~)3/'21 167+7

I

~

othc,, ex[ . . . . . . . . . . . .

299i 10.42

1.24 ~'

2D5/2

2824

5.57

7.83 d [ 4 . 6 e

2D3/?

3280

0.61

0.73 e

2).

3(}74

O. 1 1 9

0.16 ~

1961

2.48

3.7{~ d 5.%~6~

4s 2 ~LI31.~

I

I This ln\ s

,o

i F5,/2

4s 2

1370~-70 i

2}.1.

,

4s 2 Jib /

] 404±h,

! 4

i]/:

3dlO i'

479±2: ~

2442 12.O9

"q 1 / 2

1.!:,~" L . ' )

I. ~

11.8 j

i i

4>L/2 I

34~]2:,

i ~2b±I0a)20i3 Ob

]

3dlO 2.51/2

.)492

4s 2 21)~e 2

3/2J

2.87

3.6!'

3.3 ~L ~ . 2 '

0.012

f

~.4 ]

0.011Y

118±[6 c .fll~ f

(

,

],i~ #'

I

4

li

JS!~± ~4

!61i54 a

i ~jI.O

,244 I, .,,~

4S 2 "dD ) / ~,

• .,

O.91

3194

1 . b 5d

I

2.>!

q !

~'

]

2

!99~

D %/ 2

(

.i 27

I

4~ 2 2D I/2

l Kre[lman:/

~t

ZD>/2 [

at.,

Rc'~. 8.

Kt>w.~Jsgi 1:1:1 Puti ! t ; : , Meg~,i.r : <.t h i . , .gll~n ~

t I,>LI

arlij

,\~ad,!,

r;,,,

{208

ilJ,J

k~f.

12.

ttnormai[zed

ill R e r .

rcl:o~-rTatLzed }',, > b h h ! ,

Ref.

in go~. 18,

I..]gs

,,..,3 t

~ .... b

Bucka

d

Kock

I

.is'

e~ atad

C.rlis*;,

I

1.

tt

Slavenas,

j

[New, k e f .

. . . . Ref.

at.,

11. Re[.

Ric!lt,.r,

renormalized Rt-~.

i

~.

in R e f .

I.

17.

I'

17-19 are put on an absolute scale in Table 2 by normalizing the transition probability of the 3248-.A line to the same value adopted by Kock and Richter (1.39 x 10~s '), which is believed to be accurate within 10% on the basis of several lifetime measurementsd-~'~ It is seen that the values obtained in the previous experiments are generally larger than our results, although they agree with ours within a factor of about two, except for a few cases. Among the results of emission measurements, the highest accuracy (14-17%) has been assigned to those of Kock and Richter. The discrepancies between our results and those of Kock and Richter are, however, beyond the limits of experimental uncertainties. Since lifetime measurements give inherently less uncertain results, we believe that the descrepancies between our results and those of others are mainly due to systematic errors in the latter. Acknowledgement--The multi-exponential least-squares analysis was made at the ('omputation ('enter of Nagoya (mversib.

1. 2, 3. 4. 5. 6. 7. 8.

REFERENCES C H. Corliss. J. Res. Nat. Bur. Stand, 74A, 781 (1970) A. Bielski. JQSRT 15, 463 (1975). A. Kono and S. Hattori, J. Opt. Soc. Am. in press. J, Ney, Z. Physik 196.53 (1%6L L, A. Levin and B. Budick, Bull. Am. Phys. Soc. 11,455 il966). P. T. Cunningham and L. J. Link, J. Opt. Soc. Am. 57, l(t{X¿[1967L Y. Andersen, K A. Jessen, and G. S~rensen, Nucl. Instr• Methods 90, 35 (lt~7o! H. Krellmann, E. Siefart. and E. Weihreter, J. Phys. BS, 2608 (1975). ~ ~lrd, B Engman. andl Martin~on, Phvsi('aScripta 13. 10C~llgV6t

Some lifetimes and transition probabilities in Cu(1) 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

387

Yu. I. Malakhof, Opt. Spectrosc. 44, 125 (1978). H. Bucka, J. Ney, and K. P. Wirtnik, Z. Physik 202, 22 (1%7). J. Kowalski and G. Zu Putlitz, Z. Physik 208, 459 (1%8). M. Kock and J. Richter, Z. Astrophysik 69, 180 (1%8). W. F. Meggers, C. H. Corliss and B. F. Scribner, "Tables of Spectral-Line Intensities", NBS Monograph 32 (U.S. Government Printing Office, Washington D.C. 1%1). C. H. Corliss, J. Res. Nat. Bur. Stand. 66A, 497 (1%2). C. W. Allen and A. S. Asaad, Mon. Not. Roy. Astron. Soc. 117, 36 (1957). I. -Yu. Yu, Slavenas, Opt. Spectrosc. 20, 264 (1%6). P. P. Ostroumenko and V. S. Rossikhin, Opt. Spectrosc. 19, 365 (1965). B. V. Lvov, Opt. Spectrosc. 28, 8 (1970).