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44393-G6
Resolution of Transient States of Nitrile Anions via Photodissociation Action Spectroscopy
During 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 orthogonal accelerator has been designed- consisting of 7 parallel capacitor plates resistively divided to ground- and installed in the TOF. The pulsed electronics to support the acceleration grid have been tested and installed.
- a microchannel plate detector has been successfully mounted and installed in the TOF
- a hemispherical, kinetic energy analyzer has been redesigned and is currently being professionally fabricated.
- 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.)
As this is a custom instrument, there is necessity to replicate well understood systems to make certain that the instrument is functioning as designed. The well-known resonant multi-photon, and resonant 2-photon ionization spectra of neutral toluene and toluene clusters were investigated to calibrate the instrument and test the data acquisition code. Although each component was tested and worked well independent of the whole, the assembly required significant modification and early multi-photon ionization mass spectra resulted in unresolved toluene mass “blobs”. However, after a few months of careful experimentation, instrumental modification, and reprogramming; well resolved mass spectra as well as R2PI and REMPI signals were acquired.
Photodissociation action spectroscopy of cold anions is the ultimate goal of this research and therefore, the supersonic source was modified to include 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, we decided to mock up on neutral and cationic signals in order to optimize the timing delay between the pulsed valve, vaporization laser, and pulsed acceleration grid. During this time, we found some interesting differences between the R2PI and REMPI spectra of atomic neutral copper which we describe below.
As seen in the “nugget” accompanying this report, two laser scans through the same region (the top trace: R2PI, the bottom trace: REMPI) yield obviously different spectra and these differences must be due to the different sampling techniques. The laser fields used during R2PI were the doubled output of a dye laser and 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 REMPI technique utilizes only the focused output of the doubled dye laser and results in a far richer spectrum. The intense, broad feature ~31150 cm-1 is the 2 photon/3 photon ionization efficiency limit. Dye laser frequencies less than this limit require 3 photons to ionize atomic copper while frequencies above this limit require only 2 photons. Although the 3 photon ionization efficiency is clearly inferior, there are obvious resonance effects which enhance ionization. These resonances are a Rydberg series terminating at the Cu IP. Initially, it was suspected that long-lived metastable states of atomic copper were produced in the vaporization process and these resonances were 2-photon ionizing transitions from the metastable. This is not possible however; there are only a few metastable states in atomic copper and none of these would account for the resonant REMPI structure.
We assign the resonant transitions acquired through the REMPI technique as the 2P(3d10np, where n = 10-19)-2S1/2 (3d104s) Rydberg series which occurs at twice the dye laser frequency. We believe that the focused doubled dye laser output creates a superposition state (probably a superposition of the spin orbit components of the 2P stationary states, observed in R2PI) and this virtual state provides the mechanism for resonant 2-photon absorption. Here, resonance implies that the 2 photon sum matches the state energy and ionization occurs from the absorption of a third photon.
To conclude, we are rapidly approaching our goal of cold ion spectroscopy. We believe that the novel techniques which we proposed and the instrumentation which we have generated require careful experimentation and calibration. We appreciate the liberty with which the ACS-PRF type G granting program permits the study of projects not directly related to the proposed research.
