44393-G6
Resolution of Transient States of Nitrile Anions via Photodissociation Action Spectroscopy
Narrative Report
During the time of submission to the ACS PRF, my graduate students and I had nearly completed the construction of an instrument capable of generating and optically interrogating mass selected ions. A large vacuum chamber was coupled to a custom time of flight mass spectrometer and all the vacuum hardware was in place. Since that time, our research group has progressed forward as indicated by the following achievements:
- a custom Wiley-McLaren, pulsed orthogonal accelerator; consisting of 7 parallel capacitor
plates resistively divided to ground, has been installed in the custom TOF.
- a hemispherical, kinetic energy analyzer has been designed and custom fabricated. The
analyzer permits the optical study of cold ions through photodissociation action spectroscopy.
A microchannel plate detector has been mounted to the exit aperture of the sector and we are
currently studying photodissocation of cold ions.
- a National Instruments LabVIEW program has been successfully coded to control all
acquisition parameters. (This was actually a huge task to efficiently interface the lasers, pulsed
acceleration grid, pulsed valve, signal acquisition, etc. with the graduate student users.)
Photodissociation action spectroscopy of cold anions is the ultimate goal of this research and therefore, a supersonic source was constructed to entrain and jet-cool the products formed in a laser-driven vaporization plume. The focused output of an excimer (or dye) laser was timed to ablate the surface of a metal rod just as the densest portion of the expansion gas passes. The expansion gas (which may be doped with the vapor pressure of some organic) thus picks up the vaporization products and provides the environment where charged or neutral molecules may form and cool as the gas expands into vacuum. Although cold anion spectroscopy is the research goal, it was decided that neutral and cationic signals should be first acquired in order to optimize the timing delay between the pulsed valve, vaporization laser, and pulsed acceleration grid. It was also determined that this was the ideal time to test the data acquisition program to ensure routine performance of the experiment.
To date, our research has been very successful at generating metal-ligand cluster cations and we have successfully studied such clusters through photodissociation action spectroscopy. Prior to these studies, however, considerable time was spent in optimizing experimental parameters by observing resonant signals in a cold beam containing atomic copper. This has been recently published and a brief discussion of those results is presented.
As seen in the “nugget” accompanying this report, two laser scans through the same region
(R2PI and R(1+1)3PI) yield obviously different spectra and these differences are due to the different sampling techniques. The laser fields used during R2PI studies utilize doubled dye laser output overlapped with the 4th harmonic of a Nd:YAG laser. The 2 most intense transitions occur when the dye laser resonantly drives the first spin allowed transitions from ground state copper, the 2P1/2←2S1/2 and 2P3/2←2S1/2 spin orbit components. The R(1+1)3PI studies (where the (1+1) indicates a resonant state accessed by the summed energy of two photons from the same laser) resulted from copper absorption of laser radiation at intensities ~108 W cm-2. The intense features are assigned as a 2-photon Rydberg series (2D5/2←2S1/2) converging to the copper ionization limit. Bold numbers above the resonant transitions label the 3d10 nd configuration states at term energies of twice the dye laser frequency. The state, now populated via simultaneous absorption of 2 dye laser photons, ionizes through absorption of a 3rd dye laser photon which couples the prepared state to an autoionizing state. The state which facilitates the simultaneous, 2-photon absorption is a non-stationary state formed primarily by the superposition of the lowest lying copper 2P spin orbit components.
Receiving the ACS-PRF starter grant has impacted my early career. Not only through the prestige
associated with this award, but also the financial impact and liberty with which starter funds may be
spent has greatly forwarded my research agenda. A good portion of the funds was used to purchase a (used) YAG laser with which my students have established a love/hate relationship. They have learned the delicate intricacies associated with the laser head and have learned what it takes to make an older laser function consistently and routinely. Additional ACS-PRF starter funds were used to fund student trips to a regional ACS meeting to present early findings. It is my belief that such activities which require students to publicly defend their research make them into competent scientists. In conclusion, our group appreciates the ACS-PRF granting agency and the liberty with which Type-G starter funds may be dispersed.
