Simulations of multi-photon dissociation of calcium monohydride and calcium monodeuteride ions
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Janardan, Smitha Sree
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Abstract
We observe various transitions in CaH$^+$ and CaD$^+$ by resonance enhanced multiphoton photodissociation spectroscopy in a Coulomb crystal. We report the the vibronic 1$^{1}\Sigma$ v$_{0}$ $\longrightarrow$ 2$^{1}\Sigma$ v'$_{0,1,2,3,4}$ transitions of CaH$^+$ and CaD$^+$; rovibronic 1$^{1}\Sigma$ v$_{0}$,J $\longrightarrow$ 2$^{1}\Sigma$ v'$_{0,1,2,3}$,J' transitions of CaH$^+$; vibrational 1$^{1}\Sigma$ v$_{0}$ $\longrightarrow$ 1$^{1}\Sigma$ v'$_{9,10}$ transitions of CaH$^+$. This system is ideal for performing high-precision molecular spectroscopy with applications in astrochemistry and fundamental physics. These measurements are vital to future quantum logic spectroscopy (QLS) experiments and for measuring rotational state preparation of CaH$^{+}$. We used CaH$^{+}$ co-trapped with Doppler cooled Ca$^{+}$ to perform spectroscopy using photodissociation. The Ca$^{+}$ fluorescence served as a detector for dissociation rates. The broad linewidth of the laser provided an advantage for the initial search for transitions, but did not allow spectral resolution of rotational transitions. We use pulse shaping to spectrally narrow the linewidth of the laser to obtain rotational constants for the rovibronic states. We assign the observed peaks to the transition from the ground vibrational state using a theoretical model based on previous theoretical predictions. The simulation method comes in two flavors: simple and full simulations. The simple model that assumes instantaneous dissociation after resonant excitation, while the full simulations uses a matrix ordinary differential equation to account for all possibilities. Both methods convolute a tunable laser with an underlying rovibronic spectrum to find corresponding transition rates. Physical constants and information, such as transition frequency, transition dipole moments, rotational constants, and dissociation pathway, come from fitting the simulated spectrum to the experimental spectrum. The vibrational transitions matched theory well when a thinner laser linewidth is assumed. The CaH$^{+}$ vibronic peak assignments initially match theoretical predictions well. Comparisons with CaD$^{+}$ vibronic peak assignments reveal a revised assignment of the CaH$^{+}$ vibronic levels and a disagreement with CASPT2 theoretical calculations by approximately 700 cm$^{-1}$. Rovibronic peak assignments affirm the predicted rotational and distortion constants. We will describe possible dissociation paths through excited $\Sigma$ and $\Pi$ states.
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2017-11-09
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Dissertation