HIGH RESOLUTION SPECTROSCOPY
~
~
2
OF THE B A1 - X2A1 TRANSITION OF
CaCH3 and SrCH3
P. M. SHERIDAN, M. J. DICK, J. G. WANG AND P. F. BERNATH
University of Waterloo
Metal Polyatomic Molecules
• MCH3, MCCH, MSH, MNH2…
– Interest in Catalysis, Organic Synthesis, Biological Systems,
Astrochemistry…
• Not As Well Characterized As Diatomic Counterparts
• Lower Symmetry Complicates Spectra
– Simplify Analysis by Characterizing these Species in a Molecular
Jet/Laser Ablation Source
– See TC08, TC09, and TC10
Alkaline - Earth Monomethyls
• First Gas Phase Observation (Brazier and Bernath 1987)
– Low Resolution Spectra: Ca and Sr Reacting w/Methyl
Precursors
~2
~2
• CaCH3 A E – X A1 (Brazier and Bernath 1989)
– High Resolution Spectrum in Broida Oven
~2
~2
• MgCH3 A E – X A1 (Rubino, Williamson and Miller 1995)
– High Resolution Jet Cooled Spectrum
Alkaline - Earth Monomethyls
• Optical Stark Spectra CaCH3 (Marr et al 1996)
~2
~2
– Determined Dipole Moments in A E and X A1 States
• Millimeter-Wave Spectroscopy (Ziurys Group)
– Ground State Pure Rotational Spectra of Mg, Ca, Sr, and BaCH3
• Low-Lying States Not all Well Characterized at High
Resolution
~~
– Initiated a Study of the B-X Transitions of CaCH3 and SrCH3
Using Molecular Jet/Laser Ablation Techniques
Laser Ablation Source
Preamp
I2 Cell w/ PMT
Single Mode
Ring Dye Laser
Rod Rotator
Scope
Preamp
Boxcar
PMT
Gas In (1% Sn(CH3)4 in Ar)
Delay Box
Backing Pressure (100 psi)
Pulsed Valve
Power Supply
Pump
YAG
PC
3rd Harmonic
Trot ~ 4–8 K
~2
~2
B A1 – X A1 Transition
• CaCH3 and SrCH3
B2 Σ+
~
B2A1
– Prolate Symmetric Top
– C3v Symmetry
~
• B2A1 Correlates to B2S
– || type transitions
– a-dipole moment
– DK = 0
A2Π
~2
AE
• Nuclear Spin Statistics
– Rotationally Cool into Both
K" = 0 and 1 Levels
X 2Σ+
SrF
CCv
v
SrCH3
C3v
C
3v
~
X 2A1
High Resolution Spectra CaCH3 and SrCH3
~2
~ 2A - X
SrCH3 B
A1
1
~2
~2
CaCH3 B A1 - X A1
0
2
4
6
8
-1
Relative Wavenumber (cm )
10
Energy Level Diagram K = 0 Sub-Band
J
N K
3.5
F2
F1
1.5
2.5
0.5
1.5
0.5
F1
2.5
R
Q21
R
R22
P
P
Q12
• Resembles Hund’s
Case(b) 2S – Case(b) 2S
Transition
• 4 Main Branches
• 2 Satellite Branches
R
P22
R11
P
P11
• Branch Notation DNDJFi'Fi"
3.5
2.5
F1
F2
2.5
1.5
1.5
0.5
0.5
F1
• F1: J = N + S; F2: J = N – S
Energy Level Diagram K = 1 Sub-Band
J
N K
F2
5.5 F1
4.5
3.5
4.5
2.5
3.5
1.5
2.5
0.5
1.5
R
R22
Q
Q
Q22
P
R12
P
Q12
P
P22
P11
O
P12
Q
Q
Q11
R
Q21
P21
R
R11
S
R21
3.5
2.5
2.5
1.5
1.5
0.5
F1
F2
• Resembles Hund’s
Case(b) 2P – Case(b) 2P
Transition
• 6 Main Branches
• 6 Satellite Branches
~2
~2
SrCH3 B A1 – X A1 K = 0 and 1 Sub-Bands
1.5
0.5
R
2
P
2
1.5
K=0
0.5
R
1
P
1
0.5 PQ
R
12
2.5
R
P
1
K=1
1.5
1.5
Q
Q
R
12
2
5.5
P
Q
12
14786
1.5
1
P
1.5
0.5
Q
21
1.5
Q
1
14787
0.5
R 0.5
2
P
21
0.5
4.5
Q2
14788
-1
wavenumber (cm )
R
1.5
Q
21
14789
Results and Analysis SrCH3
• Data Fit to Symmetric Top Hamiltonian
~2
~2
– 108 B A1 – X A1 Transitions and Pure Rotational Transitions
– Fit Using Pickett’s Program
Parameter
~
X2A1
~
B2A1
T
0.0
14787.58135(64)
A
5.393a
5.31139(84)
B
0.193833336(24)
0.193603(14)
DN
2.14893(16) x 10-7
0.0
DNK
1.61313(77) x 10-5
5.4(1.8) x 10-5
eaa
0.0
-0.2523(21)
(ebb+ecc)/2
4.12162(51) x 10-3
-0.14879(11)
a) Fixed to Theoretical Value (Chan and Hamilton 1998)
~2
~2
CaCH3 B A1 – X A1 K = 0 Sub-Band
0.5 P
Q
12
1.5
P
2
1.5
P
1
R
0.5
Q
21
R 0.5
2
R 0.5
1
Q Branch K = 1?
16009
16010
16011
-1
Wavenumber (cm )
16012
~2
~2
CaCH3 B A1 – X A1 K = 1 Sub-Band
0.5 P
R
Q
12
1.5
1.5 P
1.5
2.5
Q
P
12
2
P
P
2
1.5
P
R 0.5
2
R 0.5
1
1
1.5 QP
21
0.5 Q
1
Q
Q , R
2
12
1
16009
0.5
Q
21
R
0.5
Q
21
R
0.5
2
R
0.5
1
16010
s
16011
-1
Wavenumber (cm )
R
1.5
21
16012
~2
CaCH3 B A1 Perturbation
-1
Relative Energy (cm )
15
10
J
F2 N 6.5
F1 7 7.5
6
~2
CaCH3 B A1State Energy Levels
5.5
6.5
5 4.5
5.5
5
4
3.5
4.5
3 2.5
3.5
0
1.5
2 2.5
0.5
1 1.5
0 0.5
K=0
~2
CaCH3 B A1 Perturbation
-1
Relative Energy (cm )
15
10
5
0
J
F2 N 6.5
F1 7 7.5
~2
CaCH3 B A1State Energy Levels
J N
7.5
6.5
7 F1
F2
5.5
6.5
6.5
5.5
6
5 4.5
5.5
5.5
4.5
5
3.5
4.5
4.5
3.5
4
3 2.5
3.5
3.5
2.5
2.5
1.5
1.5
0.5
3
6
4
1.5
2 2.5
0.5
1 1.5
0 0.5
K=0
K=1
2
1
F2 Levels
Pushed Down
~2
CaCH3 B A1 Perturbation
16020
~75 cm
-1
~2
B A1
K=1, F1, F2
~2
A E3/2 (v3=3)
K=0, F1, F2
16000
~3 cm
15960
-1
Energy (cm )
15980
15940
~2
A E1/2 (v3=3)
~2
A E3/2 (v3=0)
14780
F2
14760
14740
14720
~2
A E1/2 (v3=0)
14700
F1
-1
Results and Analysis CaCH3
• K = 0 Sub Band Fit to Symmetric Top Hamiltonian
~2
~2
– 58 K = 0 B A1 – X A1 Transitions and Pure Rotational Transitions
• Calculated Term Values for K' = 1 Levels
Parameter
~
X2A1
~
B2A1
T
0.0
16010.19538(60)
A
5.44831a
B
0.252384881(25)
DN
3.54514(29) x 10-7
DNK
1.99593(32) x 10-5
0.2532525(98)
eaa
(ebb+ecc)/2
a) Fixed to Optical Value
1.47842(41) x 10-3
-0.03604(12)
~2
Pure Precession in CaCH3 and SrCH3 B A1
• Spin Rotation Constants
– Assume Unpaired Electron in a p Orbital
– Unique Perturber Assumption
(ebb + ecc)/2 = – 2l(l+1)BAso
~ ~
DEB-A
CaCH3
CaCH3
SrCH3
SrCH3
cm-1
Measured
Calculated
Measured
Calculated
(ebb+ ecc)/2
-0.03604
-0.053
-0.14879
-0.209
– Relatively Good Agreement with Pure Precession Approximation
Structure
• Rotational Constants Fit to C3v Moment of Inertia Equations
State
CaCH3
~ 2A
X
CaCH3
~
B2A
SrCH3
~ 2A
X
SrCH3
~
B2A
rM-C (Å)
2.348
2.155
2.487
2.492
rC-H (Å)a
1.102
1.102
1.104
1.104
105.3 b
105.8 a
107.0
1
qH-C-H (º) 105.3
1
1
1
a) Fixed to DFT Calculations (Chan and Hamilton 1998)
b) Fixed to Ground State Value
~
– CaCH3 B State Structure Not Reliable
~
– SrCH3 B State M-C Bond Slight Increase: H-C-H Angle Opens
Future Work and Acknowledgements
~2A
~ 2E - X
SrCH3 A
1/2
1
13652
13654
13656
13658
-1
wavenumber (cm )
Funding: NSERC
13660