1. Laser Ablation Spectroscopy of SrCCH and SrNCBy
Michael Dick, P. M. Sheridan, Peter Bernath,
and J.- G. Wang

2. X and A state (Steimle et al.)
Group II Acetylides
MgCCH
CaCCH
SrCCH
Microwave
Ziurys et al.
A-X
(High Res.)
Tokaryk et al.
Bernath et al.
Coxon et al.
NONE
Dipole Determination
X and A state (Steimle et al.)
Other
(Low Res.)
C-X
(Soep et al.)
B′-X
(Ellis et al.) ~ ~ ~ ~
Before I begin discussing our current investigation I will quickly summarize the previous work that has been completed on the group II acetylides. As this table shows the ground states of all these molecules have been investigated by the Ziurys group using microwave spectroscopy. Also recently Dennis Tokaryk and the UNB group have looked at the A-X transition of MgCCH at high resolution. This follows previous work done by Peter and John Coxon on the A-X transition of CaCCH. As for the A-X transition of SrCCH no high resolution data existed for it, only one low resolution study by Peter had been completed to date.
As well as these investigations of the ground and A states, Steimle and coworkers have looked at the dipole moments in the X and A states using stark spectroscopy. Finally there have been other low resolution studies which looked for other higher excited states in CaCCH and SrCCH. It has been the purpose of this current study to fill in the missing gap of high resolution data for the A-X transition of SrCCH. ~ ~ ~ ~
The purpose of this work is to characterize the excited states of SrCCH at high resolution.

3. SrCCH – Electronic Transitions
For the group II metal containing polyatomic molecules, the low lying electronic transitions from the ground state may be approximated as an s → p promotion on the M atom. This results in different types of electronic transitions depending on the symmetry of the molecule.
For example an 5s → 5p promotion on Sr results in 2Σ+ - 2Π and 2Σ+ - 2Σ+
transitions in SrCCH ( point group).
Arises from a promotion of the valence 5s electron to the 5px or 5py orbitals.
Perpendicular transitions, ie. ∆λ=±1.
Before I begin discussing the results of our work, I should tell you exactly what I mean when I say A-X or B-X electronic transition within these alkaline earth acetylides
Mg, Ca and Sr all have two valence s electrons and when they form a molecule they give one of these electrons to the ligand, in this case CCH. This means they have only one s electron left. Then you can think of electronic transitions within these molecule as corresponding to approximately an s to p promotion on the Metal atom. This results in different electronic transitions depending on the point group of the molecule. For a linear molecule like SrCCH in the Cv point group this corresponds to 2Σ+ → 2Π and 2Σ+ → 2Σ+ transitions. Ie the A-X and B-X transitions.
The A-X transition corresponds to a promotion of the 5s electron to either the 5px of 5py orbitals. This corresponds to perpendicular transitions, and whence in molecular terms a 2Σ+ → 2Π transitions.
The B-X transition corresponds to a promotion of the 5s electron to the 5pz orbital. This corresponds to a parallel transition, and whence in molecular terms a 2Σ+ → 2Σ+ transitions.
Arises from a promotion of the valence 5s electron to the 5pz orbital.
Parallel transition, ie. ∆λ=0.

4. The Ablation Source Excitation Laser Ablation Laser
This slides shows a picture of our ablation source. You can now clearly see how the ablation and excitation lasers enter the chamber. The ablation through the side and the excitation through the top. You can also see how the gas in injected into the chamber through the tube at the end of the chamber. For this experiment we used a 7% mix of acetylene in Argon at a backing pressure of 100psi. What you can’t see on this slide is how the fluorescence is collected. It is collected some 15 cm down stream from the ablation region and sent through a photomultiplier, located on the back of the chamber. The signal from the PMT is then sent through the boxcar and onto the computer where it is displayed in the form of a spectrum.
Gas mix: 7% HCCH in argon at a backing pressure of 100 psi.

5. The Lasers Ar Ion Lasers Linear Dye Laser Ring Dye Laser
Before leaving the experimental setup I would like to quickly discuss the two lasers that were used for this experiment. As you can see both are Argon Ion pumped organic dye lasers. The difference between them is the resolution each has. The linear dye laser has a line width of only ~ 1cm-1 but is capable of scanning 1000 cm-1 in ~20 mins. It is this laser that we use to locate new transitions via fast survey scans. Once the transitions are located we use our coherent 699 autoscan controlled ring dye laser to look at the transitions at high resolution. This laser has a line width of ~10MHz
Laser dye - DCM

6. SrCCH – Survey Scan
This slide shows the survey scan done between ~14000 to ~15000 cm-1 looking for SrCCH. As you can clearly see both spin orbit components of SrCCH are present ~270 cm-1 apart. Also present is an Sr atomic transition and the spin orbit components for another linear molecule SrOH. The reason SrOH is in the spectrum is because of impurities in the rod, vacuum leaks and/or residual water in the chamber. SrOH forms very easily and hence is very hard to completely remove from the experiment.
SrOH didn’t interfere with the 0-0 bands of SrCCH. But it did come into play when we wanted to look at the 1-0 bands for the Sr-C stretch. As you can see the 2Π1/2 - 2Σ 1-0 transition is much weaker then the corresponding 0-0 band but not overlapped with anything else. This is not the case for the 2Π3/2 - 2Σ 1-0 component which is right underneath the 2Π3/2 - 2Σ component of SrOH. As I will show on a later slide this overlap made it impossible to analyze this transition.

7. SrCCH – High Resolution
This slide shows a comparison of the two spin orbit components of SrCCH at high resolution. As you can see the 2Π3/2 - 2Σ transition shows a larger band gab while the 2Π1/2 - 2Σ transition appears to be more compressed. Each transition however shows the expected pattern for a hunds case(a) to hunds case (b) 2Π to 2Σ transition, with 1 times B and 3 times B spaced branches. Finally the reason for the greater signal to noise ratio in the 2Π3/2 - 2Σ transition is due to better laser power in this region versus around the 2Π1/2 - 2Σ transition.

8. SrCCH – Vibrational Bands
This slide shows the vibrational bands for the Sr-CCH stretch. The figure on the left clearly shows the similarity in structure between the two 2Π1/2 - 2Σ vibrational bands but the upper spectrum shows that the signal to noise level has decreased a lot in the 1-0 band.
The figure on the left illustrates the difficulty that occurred in finding the 2Π3/2 - 2Σ 1-0 component that I mentioned earlier. As you can see it is clearly overlapped with and much weaker then SrOH. The inset further illustrates this point as you can see it is hard to distinguish SrOH lines from SrCCH lines. This meant assignment of this transition was impossible and any information about the first vibrational state of the Sr-C stretch would be derived from the 2Π1/2 - 2Σ component.
Comparison of the two vibrational bands of the 2Π1/2 - 2Σ+ transition.
Overlap of the 2Π3/2 - 2Σ+ transition with SrOH.

9. SrCCH – Assignments
This slide shows the energy level diagram for a typical Case(a) to case(b) 2Π - 2Σ transition. As you can see the upper state is split by the spin orbit effect into the two spin orbit components 2Π1/2 and 2Π3/2. Also within each spin orbit level each J or N component is split by lambda doubling. As for the ground state the only thing to notice that each N level is split by the spin rotation interaction. In fact this interaction is clearly visible on the spectrum, if you look at the lines of a R2 or Q2 branches you can see that as the J value increases they begin to widen and eventually split this is a result of the spin rotation splitting in the ground state.
Overall these splitings result in 6 branches for each spin orbit component. The resulting assignments for the 2Π3/2 - 2Σ transtion are shown in the adjacent spectrum, where you can see which directions the R, Q and P branches run.
There are 6 branches for each spin orbit component.

10. SrCCH – Molecular Constants
Molecular Constants for SrCCH (in cm-1)
As a result of these assignments the following constants were derived for SrCCH. A couple of interesting things to note about table 1 are:
The data was fit to the typical Hund’s case (a) 2Π – Hund’s case (b) 2Σ+ N2 Hamiltonian. In all 158 transitions were fit, this included the ground state transitions measured by Ziurys. We included these data to get a more accurate determination of the upper state paramaters.
We didn’t take any isotopologue data but by fixing the C-C and C-H bond lengths to those of CCH anion, a bond length for Sr-C 2.43 angstroms is determined for the A state. As expected this value is slightly smaller then the bond length of the ground state. MgCCH and CaCCH also showed this same decrease in bond length when going to the first excited state.
For the first vibrational state as I mentioned earlier only a few lines from the 2Π1/2 - 2Σ component could be assigned. This meant that only a preliminary T, B and p values could be obtained for this state. And in order to get these values we had to fix the A value to that of the v=0 state because as I said we were unable to assign any lines for the 2Π3/2 - 2Σ component . However once the fit was completed we determined the vibrational frequency for the A state to be ~345 cm-1. This value is slightly smaller then the ground state vibrational frequency reported by Peter in his low resolution work.
158 transitions were fit to a Hund’s case (a) 2Π – Hund’s case (b) 2Σ+ N2 Hamiltonian including pure rotational data using a non linear least squares fitting program.
A Sr-C bond length of 2.43 Å in the A state was calculated. As expected this value is slightly smaller than the bond length of the ground state (2.46 Å).
A Sr-C vibrational frequency of cm-1 was determined for the A state. This value is ~2 cm-1 higher than that of the X state. ~ ~ ~

11. Λ Doubling Constant - p l(l State Parameter MgCCH CaCCH SrCCH
Tokaryk et al. J. Mol. Spec 230(2005) 54-61
CaCCH
M.Li, J. Coxon, J. Mol. Spec. 176(1996)
SrCCH
This Work
A 2Π p
4.22(22)x10-4
(1)
(33) ~
The pure precession relationship for p is
l(l
Finally it is interesting to compare the lambda doubling constant p for the different alkaline earth acetylides with each other and with the predictions of the pure precession relationships.
The pure relationships say the parameter p should negative as long as the B state is the main perturber and lies above the A state. In addition the parameter should be getting larger in magnitude as you go down the group as the spin orbit parameter increases as you go from Mg to Sr and the B state gets closer. If you look at CaCCH and SrCCH, you see that the p constant of these two molecules clearly follow this trend, with CaCCH having a smaller p constant then SrCCH. MgCCH is obviously the exception and as Tokaryk noted in his paper small Mg containing molecules often don’t follow the expected trend. This is mainly because the B state is distant in Mg containing molecules and doesn’t form a pure precession partner with the A state.
You may have noticed that I have only talked about the trends in p for these molecules according to the pure precession approximation. I haven’t talked about the value of p as set out by the approximation. This is because in order to calculate it you need to know where the B state is. The location of the B state is not known for any of these molecules. It is missing!!!
The lambda doubling parameter, p, of CaCCH and SrCCH follow the pure precession relationship, while MgCCH is an exception.

12. SrCCH – The Missing B state
What one can do if you believe the pure precession for p, is to use it in reverse to predict the location of the B state. If we do that using the experimentally determined value of p we get that the B state should be at ~15160 cm-1. When we searched for the B state, we found no experimental evidence of it. As you can see we searched all the way up to almost cm-1 and still saw no experimental evidence of it. This could indicate that the B state is there but is predissociative. Other experiments in absorption are necessary to confirm this assumption. ~
Using the experimental value for p, the location of the B 2Σ+ state can be calculated to be at ~15160 cm-1.
Survey scans of this region showed no evidence of this state, possibly indicating that it is predissociative.

13. SrNC – Survey Scan
Now let’s turn our attention to some preliminary work on SrNC.
The only previous experimental evidence of SrNC is some low resolution work done by Peter. When we went to repeat this low resolution work, as you can see from the above spectrum we quickly learned that SrNC wouldn’t be quite as easy as we had hoped. Clearly this spectrum is far more confusing then it should be if SrNC was the only molecule present here. No clear pattern was observed as it had been for SrCCH. We were using methyl cyanide in an attempt to make SrNC, so clearly some of these extra peaks may be explained as arising from other larger polyatomics forming from Sr and methyl cyanide. We ran various experiments with other reactant gasses in attempt to ascertain which peaks belonged to larger polyatomics and which belonged to SrNC. Unfortunately these experiments were inconclusive therefore high resolution data was taken for the majority of these peaks. This high resolution data lends more credit to the assumption that a large polyatomic is present here.
The only previous experimental evidence of SrNC is a low resolution spectrum taken by Bernath and coworkers.
This survey scan (1% CH3CN in Ar) was far more complicated than expected. No clear pattern was observed.

14. SrNC – High Resolution Scan
This slide shows the high resolution spectrum for one of the bands shown on the last slide. Clearly the spectrum is far more congested then what would be expected for SrNC. No clear rotational pattern is discernible. This further indicates that a heavier molecule is what we are looking at.
All of the high resolution scans of all the bands in this region showed this same congested appearance.
The congested appearance of the spectra indicates that a much larger polyatomic than SrNC is being observed.
All of the high resolution scans showed a similar congested pattern.

15. Arizona State University
Acknowledgements
Funding:
NSERC
Arizona State University
Dr. Tim Steimle
Finally I would just like to thank a couple of people. Firstly I would like to thank Natural Science and Engineering Research Council of Canada for Funding. Also I would like to thank Dr. Tim Steimle for the loan of the wavemeter junior. It has been the use of this instrument that has allowed us to properly do survey scans and locate quickly the signals for all these molecules we are discussing this morning.