1. ROTATIONAL ANALYSIS OF THE C2A1 - X2A1 TRANSITION OF SrNH2~ ~
P. M. SHERIDAN , M. J. DICK, J. G. WANG AND P. F. BERNATH
University of Waterloo

2. Previous Work on Alkaline-Earth Amides
First Observation (Harris and Co-workers 1983)
Low Resolution Spectra: Ca, Sr, Ba Reacting w/ N2H4
Higher Resolution C- X Spectrum: CaNH2
Ca and Sr Alkylamide Derivatives (Bernath Group 1987)
First Laser Ablation/Molecular Beam Study of CaNH2 (A – X and B – X Whitham et al. 1990)
CaNH2 High Resolution Laser Spectroscopy:
A2B2 - X2A1 (Marr et al. 1995)
B2B1 - X2A1 and C2A1 - X2A1 (Morbi et al. 1997, 1998) ~ ~ ~ ~ ~ ~
The alkaline-earth amides were first studied by Harris and coworkers. They obtained low resolution spectra of the products of the reaction of calcium, strontium, and barium with hydrazine. They assigned the peaks they observed as belonging to the amide species of each of these metals. They also obtained a higher resolution spectrum of the C - X transition of CaNH2 and established the first molecular constants for the X and C states. This work was followed by investigations of several alkylamide derivatives of calcium and strontium by Bernath and coworkers. This work established the approximate band centers for the three lowest electronic transitions of these species. This work was then followed by Whithmam et al, who observed CaNH2 in a molecular jet for the first time. From their medium resolution spectra they were able to determine some of the molecular constants for the A and B states. Finally high resolution laser spectroscopic studies were then performed on each of the three lowest electronic transitions of CaNH2. The A – X transition was observed by Marr et al in the Steimle group in 1995 and the B – X and C – X transitions were observed by Morbi et al in the Bernath group a few years later. ~ ~ ~ ~ ~

3. Previous Work on Alkaline-Earth Amides~
CaNH2 Dipole Moment X2A1 State (Marr et al. 1995)
SrNH2 (Brazier and Bernath 2000)
A2B2 - X2A1 and B2B1 - X2A1 High Resolution Broida Oven
Low Resolution Observation of C2A1 - X2A1 Transition
Millimeter-Wave Spectroscopy (Ziurys Lab)
Li, Na, Mg, Ca, and Sr Amides and Deuterium Isotopologues
Determined r0 structures in X2A1 state; all planar
No Other Metal Amides Known ~ ~ ~ ~ ~ ~
In addition the ground state permanent electric dipole moment of CaNH2 (1.74D) was also measured in the study of the A – X transition using optical stark spectroscopy. High resolution studies of the low lying states of SrNH2 did not appear until 2002 by Brazier and Bernath. They rotationally analyzed the A – X and B- X transitions of SrNH2 from spectra recorded using a Broida oven. They also presented an improved low resolution spectrum of the C – X transition, however under Broida oven conditions and due to the nature of the C – X transition it appeared extremely congested in the Broida oven and therefore a rotational analysis was not attempted. In addition to the laser spectroscopic work, the metal amides have been investigated by millimeter-wave spectroscopy in the Ziurys lab. The rotational spectra of the metal amides of lithium, sodium, magnesium, calcium and strontium have been recorded along with their deuterium isotopologues. For each species an r0 structure was determined and each species was found to be planar in the ground electronic state. Aside from these species, the spectra of no other metal amides are known experimentally. ~

4. Focus ~ ~ C2A1-X2A1 Transition of SrNH2
Congested Spectrum: Simplify High Resolution Spectrum Using a Molecuar Jet/Laser Ablation System to Observe Under Cold Conditions
Complete High Resolution Observation of Lowest Lying States of SrNH2
Determine Geometry and Spin-Rotation Constants
The focus of this work therefore is to obtain the first high resolution spectrum and rotational analysis of the C – X transition of SrNH2. Because of the congested nature of the C – X transition under Broida oven conditions, this molecule was an excellent candidate for study in our newly rebuilt laser ablation system. Observations under rotationally cooler conditions should greatly simplify the analysis. By studying this transition we can complete the study of the 3 lowest lying states of SrNH2, the only other non linear polyatomic for which this is the case is CaNH2. Finally from the rotational analysis we can determine the fine structure constants in the C State as well as the geometry and make some comparisons to the other low lying states of SrNH2.

5. Laser Ablation Source Trot ~ 4–8 K Preamp I2 Cell w/ PMT
Single Mode Ring Dye Laser
Scope
Rod Rotator
Preamp
Boxcar
PMT
Gas In (15% NH3 in Ar)
Backing Pressure (100 psi)
Delay Box
Because Mike has already gone through the setup of our laser ablation source in detail, I will only point out the specifics relevant to this experiment. A strontium rod target was ablated using the 3rd Harmonic of a Nd YAG laser (10mJ/pulse) The reactant gas was a mixture of approximately 15% NH3 in Ar, with a nozzle backing pressure of 100psi. The probe laser was a coherent ring dye laser operating with DCM dye. The flourescence was collected by a PMT using the appropriate band pass filter and the signal was processed by a box car integrator and from there sent to the autoscan data collection system. The rotational temperature of the molecules was estimated to be in the 4 to 8 K range.
Pulsed Valve Power Supply
Pump
YAG
3rd Harmonic PC
Trot ~ 4–8 K

6. ~ ~ C2A1 – X2A1 Transition SrNH2: C2A1 Correlates to B2S+ ~
Near Prolate Asymmetric Top
Planar C2v Symmetry
C2A1 Correlates to B2S+
|| type transitions
a-dipole moment
DKa = 0; DKc = ±1, ±3
Nuclear Spin Statistics
Rotationally Cool into Both Ka" = 0 and 1 Levels ~
Lets examine the electronic structure of SrNH2. SrNH2 is a near prolate assymetric top and in the ground state the molecule is planar with C2V symmetry. The low lying electronic states of SrNH2 are shown in the figure to the right. A diagram of the low lying states of SrF is shown next to this to show how each of the states of the lower symmetry of SrNH2 correlate back to the linear limit. As you can see, the ground state of SrNH2 correlates back to a doublet sigma state and the degeneracy of the A 2Pi state of SrF has been broken in SrNH2 resulting in the A B2 and BB1 states. The C state of SrNH2 correlates back to the B doublet state of SrF. Therefore the C – X transition of SrNH2 should be parallel type transitions, and because the dipole moment lies along the a molecular axis, the Sr-N bond, the selection rules are Delta Ka = 0 and Delta Kc = +/- 1 and +/- 3. Therefore rotational transition can be grouped into Ka sub bands. Rotation about the a axis results in an exchange of the two protons and gives rise to 2 nuclear spin states. As a result, the molecules will rotationally cool into both the Ka” = 0 and Ka” = 1 levels. Therefore unlike in the CaSH and SrSH we expect transitions arising from the Ka = 1 sub band to be quite intense.

7. High Resolution Spectrum SrNH2~ ~
C2A1 – X2A1
This is the complete high resolution spectrum we recorded for the C – X transition of SrNH2. You can see a series of equally spaced lines indicating a parallel transition Because of the cold temperature of the spectrum, only low J levels are populated in each electronic state, therefore the spectrum quickly tapers off as you move away from the origin. Unlike in the spectra shown earlier of the C – X transition of CaSH and SrSH, there is a strong Q branch located in the origin gap, which arises from the Ka = 1 sub band. The spectrum apprears a bit congested, because of the similar band origin of both the Ka = 0 and 1 sub bands.

8. Energy Level Diagram Ka = 0 Sub-Band
Resembles Hund’s Case(b) 2S – Case(b) 2S Transition
4 Main Branches
2 Satellite Branches
Branch Notation DNDJFi'Fi"
F1: J = N + S; F2: J = N – S
IN order to make rotational assignments in the high resolution spectrum it is helpful to look at the energy level diagrams for the Ka subbands. This is a diagram of the Ka = 0 sub band, similar to one shown earlier for the C –X transitions of CaSH and SrSH. The transitions in this sub band resemble a hund’s case b doublet sigma – hund’s case b doublet sigma transition, giving rise to four main branches shown in blue and 2 satellite branches in red. The branch notation is as given here. Due to the unpaired electron, each of the rotational levels is split into 2 spin-rotation components labeled as F1 and F2. In the ground state the spin rotation interaction has been found to be small and positive, therefore the F1 levels lie above the F2 levels. However, in the excited state the spin rotation interaction is expected to be negative and therefore the F2 levels lie above the F1 levels.

9. Energy Level Diagram Ka = 1 Sub-Band
Resembles Hund’s Case(b) 2P – Case(b) 2P Transition
6 Main Branches
The Ka = 1 sub band is more complicated than the the K a = 0 transitions. The rotational structure of the allowed transitions resembles a Hund’s case b doublet pi - Hund’s case b doublet pi transition. There are 6 main branches, a P, Q, and R branch in each spin component, shown in blue.

10. Energy Level Diagram Ka = 1 Sub-Band
Resembles Hund’s Case(b) 2P – Case(b) 2P Transition
6 Main Branches
6 Satellite Branches
There are also 6 satellite branches possible between the spin rotation components shown in red.

11. Energy Level Diagram Ka = 1 Sub-Band
Resembles Hund’s Case(b) 2P – Case(b) 2P Transition
6 Main Branches
6 Satellite Branches
Each Branch can Exhibit Asymmetry Splitting
24 Branches Total
And finally, since SrNH2 is an asymmetric top, the K degeneracy is removed in each of these rotational levels, resulting in asymmetry splitting in each level, labeled by Kc. Therefore each branch may appear doubled in the spectrum and result in 24 total branches.

12. SrNH2 C2A1 – X2A1 Ka = 0 Sub-Band~ ~
SrNH2 C2A1 – X2A1 Ka = 0 Sub-Band
Knowing the possible transitions for the C – X transition, we could then make quantum number assignements in the high resolution spectrum. Here you can see a sub section of the Ka = 0 sub band. The 3 transitions to the red of the origin are shown. Although the spectrum appears congested we could readily pick out several series of lines on each side of the origin. The spacings within each series differed slightly, as in the larger spacing of the P22 branch or the smaller spacing in the P11 branch as a result of the spin rotation interaction in the C state being larger than in the X state. These different spacings could be used to assign each branch as either the F1 or F2 component. The greater difficulty arose in matching each series on one side of the origin to one on the other side. This was accomplished using ground state combination differences obtained from the millimeter wave transitions. This method was also used to confirm our J assignments.

13. SrNH2 C2A1 – X2A1 Ka = 1 Sub-Band~ ~
SrNH2 C2A1 – X2A1 Ka = 1 Sub-Band
For the Ka = 1 sub band, similar transition to the red of the origin are shown. In this case however, each branch exhibits an asymmetry splitting as described before. Because of the high signal to noise ratio of the spectrum, 2 small features at approximately could be assigned as the weaker sattelite OP12 branch. The observation of these satellite branches further confirmed the correctness of our assignments.

14. Results and Analysis 240 Transitions in all 30 Branches
Fit to Watson’s S-Reduced Asymmetric Top Hamiltonian
Included Pure Rotational Transitions (Pickett’s Program) ~ ~
Parameter (cm-1)
X2A1
C2A1 T
0.0
(45) A
(26)
(26) B
(85)
(96) C
(81)
(96) DN
(15) x 10-7
4.72(28) x 10-7
DNK
4.5302(18) x 10-5
eaa
(11)
0.0869(20)
ebb
(26)
(15)
ecc
(25)
(14)
DSNK
-1.84(39) x 10 –10
After assigning the C – X spectrum of SrNH2 we fit our data along with the pure rotational data to Watson’s S reduced asymmetric top Hamiltonian using Pickett’s program. Because the pure rotational data was included the spectroscopic parameters of the ground state were allowed to vary in the fit. Only a sub set of the X state parameters is shown here From this fit we were able to determine the three rotational constants of the C state as well as one centrifugal distortion constant DN. In addition, the three spin rotation constants of the C state were determined. As you can see the spin rotation constants are two orders of magnitude greater in the C state than in the X state and negative.

15. Pure Precession C2A1 CaNH2 and SrNH2~
Pure Precession C2A1 CaNH2 and SrNH2
Spin Rotation Constants
Treat Unpaired Electron in a p Orbital
Unique Perturber Assumption
eaa = 0
DEC-B ~
ebb = –2l(l+1)BAso
DEC-A ~
ecc = –2l(l+1)CAso
CaNH2
SrNH2
cm-1
Measured
Calculated
eaa
0.998
0.0869
ebb
-0.205
ecc
-0.144
Because the three spin rotation constants of the C state were determined, their values can be compared to those estimated using the pure precession approximation. IF we assume that the unpaired electron in the C state is in a p type orbital and use the unique perturber assumption, the following expressions for the three spin rotation constants can be derived. The spin rotation constants of the C states of both CaNH2 and SrNH2 are shown below. As you can see the agreement is fairly good, for ebb the measured value is about 75% of thte calculated value while for ecc the measured value is about 90% of the calculated value. IN bothe cases the sign is in agreement. For eaa which is calculated to be zero, a small almost zero value is found for SrNH2. The aggreement between the calcualted and measure values is quite good considering all of the assumption made

16. Structure
Least Squares Fit of A, B, and C Rotational Constants to Moment of Inertia Equations
Fixed rNH to Ground State Value Å
Sr-N Bond Length Decreases in Excited States
H-N-H Bond Angle Opens in C State: Unpaired Electron Most Likely Polarized Away from Sr Atom
C State Inertial Defect (D0) = amu Å2 : Planar
State
rM-N (Å)
rN-H (Å)
qH-N-H (º)
X2A1
2.256
1.021
105.5
A2B2
2.235
105.3
B2B1
2.238
105.2
C2A1
2.245
108.1 ~ ~ ~
Now that the rotational constants of the C state have been determined, a comparison of the structures of the four low lying states of SrNH2 can be made. Because only no isotopolgues were observed in the analysis of the excited states, the nitrogen- hydrogen bond length of the isotopologue was fixed to the r0 value obtained in the millimeter-wave work. Using a least squares fit of the A, B, and C rotational constants to the moments of inertia equation the metal nitrogen bond length and H-N-H bond angle was determined for each species. From this table, the strontium-nitrogen bond length is seen to decrease in the excited electronic states similar to what has been observed in the metal ligand bond lengths of the excited states of other strontium containing polyatomics such as SrOH and SrCCH. Additionally, while the H-N-H bond angle appears to close very slightly in the A and B states as compared to the X state, the bond angle opens by about 2.5 degrees in the C state as compared to the X state. This may be the result of the unpaired electron being polarized away from the ligand allowing the angle to open. Finally, the inertial defect, a measure of a molecules planarity, was calculated for the C state. The value is small and positive, indicating that the molecule is planar in the C state. ~ ~ ~

17. Future Directions ~ ~ A- X SrCH3; Ca and Sr BH4
Ba Analogs (Ti:Saph Laser) BaCCH, BaCH3, BaSH, BaNH2 …
Funding: NSERC
Finally, we would like to continue our investigations of strontium and calcium containing molecules by completing our analysis of the A-X transition of SrCH3 as well as studying the low lying transitions of Ca and Sr borohydiride for which no high resolution information exists. Additionally, with our Ti Saph laser now working we would like to expand this work to include the barium analogs of the species present by mike and myself today. Finally, I thought I would show a picture of our group member that could not make it to columbus, Dr. Jin Guo Wang who has been instrumental in getting all of this work done. Finally I would like to thank NSERC for funding and you for your attention.