The [17.0] 2Π1/2←X 2Π1/2 system of AlCa

Article abstract of DOI:10.1016/S0009-2614(00)00252-9

Laser-induced fluorescence spectroscopy has been used to study supersonically cooled AlCa. This study investigates under higher resolution (0.007 cm-1) a single band previously studied and tentatively assigned as the (0-0) vibrational transition of the [17.0] ²Δ_3/2(?)←X ²Π_1/2 system of AlCa. The resolution of the rotational structure in the present study enabled a definite assignment as a ²Π_1/2←²Π_1/2 transition. Analysis of the spectrum gives B₀′=0.096685(19) cm-1, (p+2q)′=-0.013078(370) cm^-1, and B₀″=0.105518(20) cm^-1. These convert to ground and excited state bond lengths of r₀″=3.14942(30) and r₀′=3.29014(32) A?, respectively.

Full text of DOI:10.1016/S0009-2614(00)00252-9

7 April 2000
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Chemical Physics Letters 320 2000 303–306
www.elsevier.nlrlocatercplett
2
The 17.0 P1r2 X 2 P1r2 system of AlCa
w
x
1
Jacqueline C. Fabbi , Jon D. Langenberg, Michael D. Morse)
UniÕersity of Utah, Department of Chemistry, 315 S. 1400 East, Room 2020, Salt Lake City, UT 84112-0850, USA
Received 16 November 1999; in final form 11 February 2000
Abstract
Laser-induced fluorescence spectroscopy has been used to study supersonically cooled AlCa. This study investigates
Ž
.
Ž
.
under higher resolution 0.007 cm-1 a single band previously studied and tentatively assigned as the 0-0 vibrational
X 2P1r2 system of AlCa. The resolution of the rotational structure in the present study
w
x 2
Ž .
transition of the 17.0 D3 ?
r2
enabled a definite assignment as a 2P1
P1r2 transition. Analysis of the spectrum gives B0 s0.096685 19 cm-1,
2
X
Ž
.
r2
X
Y
y
1
-1
Ž
.
Ž
.
Ž .
pq2q sy0.013078 370 cm , and B0 s0.105518 20 cm . These convert to ground and excited state bond lengths
Y
X
˚
Ž .
Ž
.
of r0 s3.14942 30 and r0s3.29014 32 A, respectively. q 2000 Elsevier Science B.V. All rights reserved.
w x
1. Introduction
formed on diatomic AlCa 2 . Ab initio calculations
were also performed to ascertain the potential energy
curves of seven low-lying states of AlCa. In the
resonant two-photon ionization study of AlCa in a
supersonic jet, the ground state of the molecule was
determined to be of 2 Pr symmetry, with a bond
w x
As one of a series of experiments 1 on the 3d
transition-metal aluminides, AlCa was investigated
w x
in 1994 by Behm, Morse, Boldyrev, and Simons 2 .
Although calcium is an alkaline earth element, AlCa
was investigated as an example of an aluminide of
the same period without d electrons. Because of the
difficulty in promoting calcium to a 4s13d1 or a
4s14p1 configuration, the AlCa molecule provides a
prototype for understanding the chemical bonding
between aluminum and transition metals with stable
3dn4s2 configurations.
˚
length of 3.1479"0.0010 A. This ground state orig-
2
1
Ž
inates from the interaction of ground state Al 3s 3p ,
2 P1r2,3r28 and Ca 4s , S0 , with the Al 3p electron
in the pp orientation. Transitions to four excited
states were observed in the wavenumber range from
13 500 to 17 900 cm-1, with rotational analysis suc-
cessfully performed on transitions to three of the
four band systems. This rotational analysis yielded
symmetries and bond lengths for the 13.5 Pr , the
15.8 S, and the 17.6 D3r2 excited states. Transi-
x 2
Ž .
? excited state were not
1
2
.
Ž
.
The investigation by Behm et al. provided the
first experimental and theoretical studies to be per-
w
x 2
w
x 2 x 2
w
w
tions to the 17.0 D3r2
successfully rotationally resolved, but a tentative as-
signment was made based on the apparent intensities
of the rotational branches at a resolution of 0.04
cm-1. The present study was initiated to resolve the
)
Corresponding author. Fax: q1-801-581-8433; e-mail:
morse@chemistry.chem.utah.edu
1 Present address: Artel, 4750 Wiley Post Way, Salt Lake City,
UT 84116-2878, USA.
0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.
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.
PII: S0009-2614 00 00252-9

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)
304
J.C. Fabbi et al.rChemical Physics Letters 320 2000 303–306
w
x 2
Fig. 1. Rotationally resolved 0-0 band of the 17.0 P1r2 X 2 P1r2 system of AlCa.
uncertainty in this assignment by investigating a
2. Experimental
w
x 2
Ž .
transition to the 17.0 D3r2 ?
state under much
.
using laser-induced
-1
Ž
higher resolution 0.007 cm
Laser-induced fluorescence spectroscopy of AlCa
entrained in a supersonic jet was performed using an
fluorescence with a cw ring dye laser employed for
w x
excitation.
instrument that has been previously described 3 .
w
x 2
Fig. 2. R band head of the 0-0 band of the 17.0 P1r2 X 2 P1r2 system of AlCa.

(
)
J.C. Fabbi et al.rChemical Physics Letters 320 2000 303–306
305
Ž
was excited by an argon ion Coherent, Innova 200-
15 pumped cw ring dye laser Coherent 899-21,
The AlCa molecules were formed by pulsed laser
Ž
.
Ž
.
.
Ž
ablation Nd:YAG, 1064 nm of an AlCa 2:1 alloy
.
metal disk, followed by supersonic expansion in
f0.5 MHz linewidth which crossed the molecular
Ž
.
helium carrier gas 120 psi . The metal target used in
beam at right angles 1 cm downstream from the exit
orifice. The fluorescence was detected using a Ham-
mamatsu R3896 photomultiplier tube after being dis-
persed by a Spectral Energy GM252 monochromator
w x
the previous study 2 of AlCa was used in the
present investigation. From the point of vaporization,
the metal atoms and molecules were entrained in a
pulse of carrier gas and traveled through a 6 cm long
channel with a 2 mm exit orifice before expansion
into vacuum. The resulting jet-cooled molecular beam
Ž
.
3.3 nmrmm linear dispersion , with an entrance slit
width of 0.50 mm, and an exit slit width of 1.0 mm.
Fluorescence in the 0-1 band at 595 nm was moni-
Table 1
Line positions of the 0–0 band of the 17.0 P1r2 X 2 P1r2 system of AlCaa
w
x 2
Label
JY
Frequency
Residualb
Label
JY
3.5
1.5
1.5
2.5
2.5
3.5
3.5
4.5
4.5
5.5
5.5
6.5
6.5
7.5
7.5
Frequency
Residualb
y1
y1
y1
y1
Ž
cm
.
Ž
cm
.
Ž
cm
.
Ž
cm
.
Ra
Rb
Ra
Rb
Ra
Rb
Rb
Ra
Rb
Ra
Rb
Ra
Rb
Ra
Rb
Ra
Rb
Rb
Ra
Ra
Rb
Ra
Rb
Ra
Ra
Qab
Qba
Qab
Qba
Qab
Qba
0.5
0.5
1.5
1.5
2.5
2.5
3.5
4.5
4.5
5.5
5.5
6.5
6.5
7.5
7.5
8.5
8.5
12.5
13.5
14.5
14.5
15.5
15.5
16.5
17.5
0.5
0.5
1.5
1.5
2.5
16995.7103
16995.6888
16995.8876
16995.8495
16996.0454
16995.9919
16996.1192
16996.3035
16996.2269
16996.4063
16996.3176
16996.4904
16996.3901
16996.5591
16996.4447
16996.6093
16996.4827
16996.4531
16996.5899
16996.5335
16996.3277
16996.4586
16996.2415
16996.3663
16996.2547
16995.4139
16995.4139
16995.3967
16995.3713
16995.3621
16995.3175
y0.0053
y0.0011
y0.0012
y0.0009
0.0010
y0.0014
0.0007
0.0010
0.0009
0.0013
0.0017
0.0005
0.0020
0.0020
0.0021
0.0026
0.0032
0.0028
0.0005
0.0005
y0.0020
y0.0003
y0.0014
y0.0008
y0.0029
y0.0055
0.0079
y0.0030
y0.0014
y0.0002
y0.0043
Qab
Pa
Pb
Pa
Pb
Pa
Pb
Pa
Pb
Pa
Pb
Pa
Pb
Pa
Pb
Pa
Pb
Pa
Pb
Pb
Pa
Pb
Pa
Pb
Pa
Pb
Pa
Pb
Pa
16995.3099
16995.1003
16995.0901
16994.8716
16994.8457
16994.6198
16994.5813
16994.3537
16994.3023
16994.0694
16994.0057
16993.7694
16993.6924
16993.4517
16993.3620
16993.1148
16993.0122
16992.7606
16992.6467
16992.2648
16992.0003
16991.8615
16991.5924
16991.4372
16991.1605
16990.9958
16990.7179
16990.5394
16990.2549
16990.0646
0.0027
y0.0020
0.0001
0.0005
y0.0004
y0.0024
y0.0033
y0.0020
y0.0031
y0.0021
y0.0028
y0.0002
y0.0016
0.0016
0.0002
0.0019
0.0003
0.0025
0.0023
0.0056
0.0049
0.0052
8.5
8.5
9.5
9.5
10.5
11.5
11.5
12.5
12.5
13.5
13.5
14.5
14.5
15.5
15.5
0.0049
0.0014
y0.0015
y0.0018
y0.0009
y0.0023
y0.0031
y0.0036
Pb
2.5
a
Rotational constants and band origin were determined by a least-squares fit of the measured line positions to the formula
X
X
X
Y
Y
Y
X
w x
XŽ
.
Ž
Ž
.XŽ
.
Ž
YŽ
.
Ž
.YŽ
.
Ž .
6 nsn0XqB J J q1 .1r2 pq2q J q1r2 yB J J q1 "1r2 pq2q J q1r2 ,yielding the values B s0.096685 19 ,
Y
Y
pq2q sy0.013078 370 , B s0.105518 20 , pq2q sy0.000402 367 , and n0 s16995.41932 50 cmy1, with the 1s error
Ž
.
.
Ž
.
.
Ž
.
Ž .
limit in units of the last reported digit provided in parentheses after each value. These rotational constants convert to bond lengths of
Y
X
27
r0 s3.14942"0.00030 A and r0 s3.29014"0.00032 A for the dominant 96.9% isotopic modification, Al 40Ca.
˚
˚
Ž
.
b
Defined as n–ncalc
.

(
)
306
J.C. Fabbi et al.rChemical Physics Letters 320 2000 303–306
X
Ž
.
tored to avoid detecting scattered excitation laser
light. The signal was processed by gated integration
using a 10 ms gate width before being digitized and
recorded using an IBM 386 compatible computer.
Transmission fringes from a 250 MHZ confocal
The fit yielded the values B0 s0.096685 19 ,
X
Y
Ž
.
Ž
.
Ž .
pq2q 0 sy0.013078 370 , B0 s0.105518 20 ,
Y
and pq2q 0 sy0.000402 367 cm-1, with the
number in parentheses giving the 1s error limit from
the fit. The ground state rotational constant, BY0,
agrees well with that determined in the previous
Ž
.
Ž
.
etalon were used to linearize the spectrum in fre-
´
w x
quency, with calibration accomplished by compari-
study by Behm et al. 2 , but was determined with
much greater precision in the present investigation
son of iodine absorption lines to the iodine absorp-
w
x
tion atlas of Gerstenkorn and Luc 4,5 . A Doppler
due to the higher resolution obtained. From these
Y
˚
limited FWHM resolution of approximately 230 MHz
values, r0 values of r0 s3.14942"0.00030 A and
X
-1
˚
Ž
.
Ž
0.007 cm was obtained for the spectrum of AlCa
r0 s3.29014"0.00032 A are obtained 1s error
2
.
molecules in the jet-cooled beam.
limits . No lambda doubling in the X P1r2 ground
state was observed in the previous study 2 , and it is
apparent that even with the higher resolution af-
w x
3. Results and discussion
forded in the present investigation the ground state
Y
Ž
.
lambda doubling constant, pq2q , is not statisti-
0
Fig. 1 displays the rotationally resolved 0-0 band
cally significant. The rather large splitting observed
of the 17.0 P1r2 X 2 P1r2 system as recorded in
the present investigation. The presence of an R
branch band head demonstrates that the bond length
increases upon electronic excitation, and this is con-
firmed by the rotational constants obtained in this
investigation. Because of the band head, the R branch
region of the spectrum is particularly congested, but
the cw ring laser provided sufficient resolution to
assign the lines, as is illustrated in the expanded
w
x 2
in the rotational lines is almost entirely due to lambda
w
x 2
doubling in the excited 17.0 P1r2 state. The elec-
tronic state or states that are responsible for this
large lambda doubling parameter are at present un-
known.
Acknowledgements
We gratefully acknowledge research support from
the National Science Foundation under grant number
CHE-9626557. Acknowledgment is also made to the
donors of the Petroleum Research Fund, adminis-
tered by the American Chemical Society, for partial
support of this work. The authors also thank Dr. Jane
Behm for her helpful suggestions concerning the
production of the AlCa molecule, and Drs. Benoit
Simard and Joel Harris for aid in instrument design.
Ž
.
view shown in Fig. 2. The observation of the R 0.5 ,
Ž
.
Ž
.
Q 0.5 , and P 1.5 lines leads to a definite assign-
ment as an Vs0.5 Vs0.5 transition, and this in
2
turn implies that the band corresponds to a P1r2
2 P1r2 transition. The tentative assignment of
2D3r2 X 2 P1r2 obtained in the resonant two-pho-
w x
ton ionization study 2 is shown to be incorrect with
the higher resolution available in the present study.
Line positions and residuals from the fit of the lines
w x
to the standard spectroscopic model 6 are listed in
Table 1, with the resulting spectroscopic constants
given in the footnotes of the table. As can be seen
clearly in the P branch, the complexity of the spec-
trum results from a significant splitting of each P,
Q, and R line into two lines due to lambda doubling
in the ground and excited states. This effect was
included in the fit of the spectrum, yielding lambda
doubling parameters for both states. Because it is not
known which of the levels correspond to e and f
parity, the levels have been labeled a and b, with a
being the higher energy level in the excited state.
References
w x
1
J.M. Behm, M.D. Morse, SPIE Int. Soc. Opt. Eng. 2124
Ž
.
1994 388.
w x
2
J.M. Behm, M.D. Morse, A.I. Boldyrev, J. Simons, J. Chem.
Phys. 101 1994 5441.
Ž
.
w x
3
J.C. Pinegar, L. Karlsson, J.D. Langenberg, Q. Costello, M.D.
Morse, J. Chem. Phys., submitted.
w x
4
S. Gerstenkorn, P. Luc, Atlas du Spectre d’Absorption de la
Molecule d’Iode, CNRS, Paris, 1978.
´
w x
Ž .
5
S. Gerstenkorn, P. Luc, Rev. Phys. Appl. 14 1979 791.
w x
Ž .
J.M. Brown, A.J. Merer, J. Mol. Spectrosc. 74 1979 488.
6