1. Laser Excitation Spectroscopy of SrSH and CaSHBy
Michael Dick, P. M. Sheridan, Peter Bernath,
and J.- G. Wang

2. Motivation – Ca and Sr Polyatomics
Oxides, halides and hydroxides containing Ca and Sr have been well studied.
Ca and Sr polyatomics are not as well characterized but are interesting:
Light, few isotopes, main isotopes have no nuclear spin.
Metals ablate easily and form molecules readily.
Follow the progression of electronic states as the symmetry of the ligand decreases.
SrF
Diatomic
SrCCH
Linear
SrCH3
Symmetric Top
SrNH2, SrSH
Asymmetric Top
To begin, since this is the first talk of 3 in a row by our group on Ca and Sr polyatomics I thought it would good idea to give you a feel for our motivation behind this work. As most of you know a lot of work has already been done on Ca and Sr containing molecules. This includes investigations of both the diatomics and smaller polyatomics. The reason a lot of work has been done on these molecules is because Sr and Ca are good for making molecules. This is because they are light, have few isotopes and the main isotopes have little or no nuclear spin. This makes there spectra somewhat simpler. In addition if you are looking to make these molecules in an ablation source as we are, both Sr and Ca ablate and form molecules easily.
We are looking to expand the investigation of Ca and Sr containing molecules beyond the diatomics into more small polyatomics. One reason for this interest is because as the number of molecules that have been investigated expands you can begin to trace the progression of given electronic states as the symmetry of the ligand decreases. For example, you can start with the diatomic SrF, which has been previously observed and then move onto the linear polyatomic SrCCH. From there you can move into the symmetric tops with SrCH3 and finally asymmetric tops like SrNH2 and SrSH. Hopefully we will give you glimpse into this progression in the next few talks this morning.
As for previous work, the ground states of the majority of these molecules have been investigated using microwave spectroscopy, but little or no information exists on the excited states.
While the ground states for many of these polyatomic molecules have been examined, little or no information exists about the excited states.

3. ~ ~ CaSH/SrSH A-X Electronic Transition:
s → pΠ (in plane) promotion on the metal atom - perpendicular transition
Appearance like a Hund’s case (a) 2 Π - Hund’s case (b) 2Σ transition.
Previous Work:
CaSH:
Low resolution by Bernath and coworkers.
High resolution by Bernath and coworkers
SrSH:
This slide shows the correlation diagram between the diatomic CaF or SrF molecules in the Cv point group to the bent CaSH or SrSH in the C2v point group. As you can see the main difference is that the A2Π state splits into two electronic states one with A’ symmetry and the other with A’’ symmetry.
Specifically if, we look at the A-X transition we see that this electronic transition can be thought of as corresponding to a s-pΠ in plane promotion on the metal atom. This means it is a perpendicular transition and hence should have the appearance of a Hund’s case (a) 2 Π - Hund’s case (b) 2Σ transition of a linear molecule.
As for the previous work for SrSH and CaSH, Peter has seen the A-X transitions for both at low res, and he has also looked at the CaSH A-X transition at high res.

4. ~ ~ CaSH/SrSH B-X Electronic Transition:
s → pΠ (out of plane) promotion on the metal atom - perpendicular transition
Appearance like a Hund’s case (a) 2Π - Hund’s case (b) 2Σ transition.
Previous Work:
CaSH:
Low resolution by Bernath and coworkers.
High resolution by Steimle and coworkers, including Stark spectroscopy.
SrSH:
So if we look at the next possible electronic transition, the B-X transition, we see that it can be thought of as corresponding to a s-pΠ out of plane promotion on the metal atom on the metal atom. Again as in the A-X transition we see that this corresponds to a perpendicular transition and hence should have the appearance of a Hund’s case (a) 2 Π - Hund’s case (b) 2Σ transition of a linear molecule.
As for previous work again both the B-X transitions of SrSH and CaSH have been seen by Peter at low resolution. However in this case Tim Steimle has looked at the B-X transition of CaSH at high resolution. This included dipole determinations for both the B and X states via stark spectroscopy.

5. ~ ~ CaSH/SrSH C-X Electronic Transition:
s → pσ promotion on the metal atom - parallel transition.
Appearance like a Hund’s case (b) 2Σ - Hund’s case (b) 2Σ transition.
Previous Work:
CaSH:
Low resolution by Bernath and coworkers.
SrSH:
No high resolution work!
Finally the last possible s-p promotion is an s-pσ promotion this corresponds to the C-X transition. Unlike A-X or B-X this results in a parallel transition and hence will have the appearance of a Hund’s case (b) 2Σ - Hund’s case (b) 2Σ transition of a linear molecule.
The previous work for this transition is somewhat more limited with no high resolution analysis having been completed. It is with these C-X transitions that we decided to begin our investigations of CaSH and SrSH.

6. The Laser Ablation Source
Preamp
I2 Cell w/ PMT
Single Mode Ring Dye Laser
Scope
Rod Rotator
Preamp
Boxcar
PMT
Delay Box
Pulsed Valve Power Supply
Pump
Before I begin talking about the specifics of our spectra, I will quickly mention the experimental setup used for these experiments. We use the typical ablation source setup where the metal rod is ablated by the 3rd harmonic of a pulsed YAG laser. The ablated metal atoms are then carried into the reaction chamber by the expansion of the reactant gas. In the case of this experiment the gas mix used was a was a 7.5% H2S in Argon at a backing pressure of 100 psi. This gives us rotational temperatures of ~4-8K. Once inside the chamber we interrogate the newly formed molecules about 15 cm downstream from the point of ablation with our dye lasers, causing fluorescence. This fluorescence is then collected by the PMT, sent through the preamp, onto to the boxcar and finally the computer where it is combined with our I2 calibration signal and displayed in the form of a spectrum.
YAG
3rd Harmonic PC
Trot ~ 4–8 K
Gas mix: 7.5% H2S in argon at a backing pressure of 100 psi.

7. ~ ~
CaSH – C-X
This slide shows the overall spectrum of the CaSH C-X transition. As you can see it has the expected appearance of a Hund’s case (b) 2Σ - Hund’s case (b) 2Σ transition of a linear molecule with a well defined origin gap and P and R branches running away from each other.
Expected Hund’s case (b) 2Σ - Hund’s case (b) 2Σ structure.

8. ~ ~
SrSH – C-X
This slide shows the overall spectrum of the SrSH C-X transition. As you can see like the CaSH C-X transition it has the expected appearance of a Hund’s case (b) 2Σ - Hund’s case (b) 2Σ transition of a linear molecule with an origin gap and P and R branches running away from each other. The differences with CaSH are that the origin gap is slightly smaller as are the spacings between individual rotational lines. Both of these differences can be attributed to the smaller B value expected for SrSH in comparison with CaSH.
Expected Hund’s case (b) 2Σ - Hund’s case (b) 2Σ structure.

9. C-X Energy Level Diagram~ ~
C-X Energy Level Diagram
Near Prolate Asymmetric Top:
a – type transitions:
∆Ka=0, ∆Kc=±1
Diagram shows only Ka =0 subband. Other Ka≠0 subbands possible, but weaker due to cooling.
Ka=0 subband will appear like a Hund’s case (b) 2Σ - Hund’s case (b) 2Σ transition.
Each rotational level is split into F1 and F2 components by the spin-rotation interaction.
Before beginning to discuss the assignments of these spectra it is always good to examine the energy level diagram. This diagram shows only the Ka=0 levels. I have neglected to draw the Ka=1 levels because the majority of the transitions observed in the previous spectra involve only the Ka=0 levels. Transitions involving the Ka≠0 levels are far less abundant because the Ka=1 levels are less populated due to rotational cooling in the ablation source.
As you can see the Ka=0 levels alone look very much like the levels of a Hund’s case (b) 2Σ state, with each N level being split by the spin rotation interaction.
The C-X transition is an a-type transition meaning the selection rules on Ka and Kc are ∆Ka=0 and ∆Kc=±1. These selection rules, along with the typical ∆J and ∆N selection rules results in 6 branches. The same six branches you get for a a Hund’s case (b) 2Σ - Hund’s case (b) 2Σ transition.

10. CaSH/SrSH – Ka=0 Subband
This slide shows the assignments of Ka=0 levels derived from the previous energy level diagram, with the satellite branches being showed in red and the main ones in blue. What is most interesting to note here is the variation in spacings of the branches between the two molecules. For example if you look at the P branches of SrSH you see that the P1 branch spacings between rotational lines are small while the spacing between P2 lines is large. On the other hand in CaSH the P1 branch is the skinny one while P2 is the fat one. This clearly indicates that something has changed in the spectra from one molecule to the next. What has changed? Well, if the spin rotation is small in the ground state as it is expected to be here, then the relative spacings in the F1 branches vs. the F2 branches is a measure of the size and sign of the spin rotation interaction. So clearly the sign of the spin rotation interaction has flipped when going from SrSH to CaSH. I will come back to this in just a minute.
CaSH
SrSH
SrSH - P1/R2 branches smaller spacing, P2/R1 branches larger spacing
CaSH - P1/R2 branches larger spacing, P2/R1 branches smaller spacing
Indicates sign change in spin rotation interaction.

11. CaSH/SrSH – Ka=1 Subband
This slide again shows the same spectra I just discussed. However, what I have done here is highlight all the extra lines as I am sure you are wondering what they are. Well they are the Ka=1 transitions that I mentioned would be weaker a couple of slides ago. As you can see the signal to noise ratio was better in the CaSH spectra making the Ka=1 lines easier to identify. Unfortunately we have yet to assign these transitions for either molecule, we plan on recording more spectra here in order to improve the signal to noise ratio and assign the Ka=1 lines and make the assigments easier. What this lack of Ka=1 data does mean is we are quite limited on the number of molecular parameters we can determine for each of these molecules, as I will show on the next slide.
CaSH
SrSH
Red arrows indicate unassigned lines likely arising from Ka = 1 subband.
Extra lines more apparent in CaSH due to increased signal to noise ratio.

12. CaSH/SrSH – Molecular Constants~
C state molecular constants for CaSH and SrSH ~ ~
Parameter (cm-1)
CaSH C 2A′
SrSH C 2A′ T
(64)
(57) A
Fixed to ground state value
(B+C)/2
(15)
(8)
εaa
Fixed to zero
(εbb+εcc)/2
(14)
(10)
Ka=0 subband and pure rotational data fit to Watson’s S-Reduced asymmetric top Hamiltonian (Pickett’s Program).
The pure precession relationships indicate that (εbb+εcc)/2 should be negative.
CaSH appears to be following this relationship, while SrSH does not. The C state of SrSH is perturbed.
This table summarizes the molecular constants we have determined for the C state. Firstly as you can see we were unable to determine A or eaa this is because in order to determine these parameters you have to have Ka=1 information. What we have determined is an average value of B and C which as you can see it smaller as expected for SrSH then CaSH. Also we have determined an average value of the spin rotation interaction ebb+ecc/2. This is where the data gets interesting as you can see as the spectra indicated the sign of the spin rotation parameter has flipped in going from CaSH to SrSH. The pure precession relationships state that as long as the C state is above both the A and B states the spin rotation parameter should be negative in the C state. Clearly CaSH is following the expected trend while SrSH is not. This indicates that the C state in SrSH is somehow perturbed. Before I begin to speculate as to the nature of this perturbation we need to discuss the other electronic transitions. ~

13. B-X Energy level diagram~ ~
B-X Energy level diagram
c – type transitions:
∆Ka=±1, ∆Kc=0,±2
Diagram shows a Ka(1-0) subband
Ka(1-0) subband will appear like a Hund’s case (a) 2Π - Hund’s case (b) 2Σ transition.
The F1 and F2 components in the B state will exhibit a large splitting due to εaa (~ cm-1)
Each Ka=1 rotational level is further split by asymmetry doubling. ~
In addition to our work on the C-X transitions of SrSH and CaSH we have also examined the A-X and B-X transitions of SrSH at high res. This slide shows the energy level diagram and possible transitions for the B-X transition. The B-X transition is a c-type transitions meaning the selection rules on Ka and Kc are ∆Ka=±1 and ∆Kc=0. This diagram shows only the the Ka=0 to Ka=1 transitions. These transitions appear like a Hund’s case (a) 2Π - Hund’s case (b) 2Σ transitions of a diatomic or linear molecule. Like in Hund’s case (a) 2Π states the B state has been split into the two spin components by the spin rotation interaction and 6 branches result in each spin component. The pure precession relationships indicate that the size of this splitting should be ~ cm-1 for the B state. Finally in addition to the spin rotation splitting each rotational level of the B state should be further split by asymmetry splitting.

14. ~ ~
SrSH – B-X
This slide shows the high resolution scans taken of the two spin components of the B-X transition of SrSH. As you can see they each show the expected Hund’s case (a) 2Π - Hund’s case (b) 2Σ transition pattern with 1B and 3B spaced branches and the F2 component having a smaller origin gap then the F1 component. In addition you can see that they are spaced by ~-15 cm-1 since the F1 component is below the F2 component . This spacing is approximately equal to the expected spacing of ~19.7 cm-1 laid out by the pure precession relations.
Each spin component has the expected Hund’s case (a) 2Π - Hund’s
case (b) 2Σ appearance with 1B and 3B spaced branches.
The approximate value of εaa(~ -15 cm-1) is close to that predicted by the pure precession relation.

15. SrSH – Assigned Spectrum
This slide shows the assignment for the F1 component of the B-X transition of SrSH. As you can see all 6 branches of this transition have been identified and confirmed using lower state combination differences. Unfortunately I can’t show you a similar slide of the F2 component, nor can I quote any molecular constants for the B state. This is because we are still assigning the F1 component. So clearly this B-X transition is very much a work in progress. So finally let me just finish up my talk by talking about the final work in progress the A-X transition of SrSH.
Lower state combination differences confirm the above assignments.
The F2 component has not yet been assigned.

16. A-X Energy level diagram~ ~
A-X Energy level diagram
b – type transitions:
∆Ka=±1, ∆Kc=±1
Diagram shows a Ka(1-0) subband
Ka(1-0) sub band will appear like a Hund’s case (a) 2Π - Hund’s case (b) 2Σ transition.
The F1 and F2 components in the A state will exhibit a large splitting due to εaa (~ 19.7 cm-1)
Each Ka=1 rotational level is further split by asymmetry doubling. ~
This slide shows the energy level diagram and possible transitions for the A-X transition. The A-X transition is a b-type transitions meaning the selection rules on Ka and Kc are ∆Ka=±1 and ∆Kc=±1. This diagram shows only the the Ka=0 to Ka=1 transitions. Like for the B-X transition, hese transitions appear like a Hund’s case (b) 2Π - Hund’s case (b) 2Σ transitions of a diatomic or linear molecule. Like in Hund’s case (b) 2Π states the A state has been split into the two spin components by the spin rotation interaction. In this case this interaction is again described by eaa. The pure precession relationships indicate that the size of this splitting should be ~ cm-1 for the A state. Finally again in addition to the spin rotation splitting each rotational level of the A state should be further split by asymmetry splitting.

17. ~ ~
SrSH – A-X
This slide shows the entire high resolution spectrum of the A-X transition. Again as expected each spin component has the structure of a Hund’s case (a) 2Π - Hund’s case (b) 2Σ transitions with 1B and 3B spaced branches. What is interesting to note here is how much closer the two spin components are in comparison with the B-X transition. Here they are only spaced by 5cm-1, much closer then the expected 19.7 cm-1 as dictated by the pure precession approximation. This indicates that the A state is somehow perturbed. But by how and by what state.
The approximate value of εaa(~5 cm-1) is not close to that predicted by the pure precession relation(~19.7 cm-1).

18. Perturbation Summary ~
The εaa value of the A state of SrSH is reduced from ~19.7 cm-1 (pure precession) to ~5cm-1. A similar reduction was also observed in the A state of CaSH.
Reduction in CaSH attributed to orbital mixing of the A and C states.
This may contribute to the change in sign of (εbb+εcc)/2 in the C state of SrSH.
The B state of SrSH appears to be less perturbed as the calculated
εaa(-19.7 cm-1) is close to the observed value (-15 cm-1)
More work is needed to fully understand these various perturbations. ~ ~ ~ ~
One conclusion you can draw is that the A state is being perturbed by the C state and vice versa. This is not unexpected since the A and C states have the same symmetry. Also a similar interaction was observed by Peter in his work on the A-X transition of CaSH. There he saw that the eaa constant was reduced from cm-1 to cm-1 because of a orbital mixing of the A state with the C states. This mixing is beyond the predictions set out by the pure precession approximation and may explain why eaa is reduced so much in the A state. This mixing may also explain the change in sign of the spin rotation parameter ebb+ecc/2 in the C state of SrSH. If the C state now has some A state character in it, it will have some eaa spin rotation to account for, enough to drive ebb+ecc/2 positive if unaccounted for. At this time the B state appears to be unperturbed as the calculated eaa value is close to the observed value. Clearly more work will be needed to fully understand these perturbations. ~