Vibrational and Vibronic Spectroscopy

Ultraviolet Photodissociation (UVPD) Spectroscopy

Spectroscopy is our most accurate tool

for determining molecular structure. In the Hopkins lab, we couple laser spectroscopy with ion trapping instruments to conduct action spectroscopy experiments whereby we monitor molecular/cluster fragmentation as a function of laser wavelength. Pictured on the right are the vibronic spectra of protonated quinoline (top) and protonated isoquinoline (bottom), which we separated using differential mobility spectrometry prior to conducting ultraviolet photodissociation (UVPD) spectroscopy. There are clear differences between the spectra of the two isomers.

Introducing UVPD as an additional, orthogonal tool for analyte characterization is useful in cases where the identify of the analyte is unknown, such as when trace components are isolated from complex mixtures or when multiple different molecular geometries are produced from a single neutral parent molecule during ionization. A textbook example of the latter is para-aminobenzoic acid, which can protonate at either the amino moiety or at the carboxylic acid moiety during electrospray ionization. Additional details can be found at this link.

Infrared Multiple Photon Dissociation (IRMPD) Spectroscopy

We conduct similar experiments using infrared lasers to probe the vibrational spectra of mass-selected molecular ions. By coupling an ion trap mass spectrometer with an infrared free electron laser (IR-FEL), we can monitor ion fragmentation efficiency as a function of laser wavelength to generate an IRMPD spectrum. For example, the image shown below plots the IRMPD spectrum for deprotonated 3,3,3,2-tetrafluoropropionic acid. Panel A shows the spectrum when we monitor the total fragmentation efficiency of the ions, whereas panels B and C show the spectrum when we monitor the CO₂ loss channel and the FCO₂^- production channel, respectively. Panel D shows the computed vibrational spectrum for the parent ion.

Instrumentation

My research relies heavily on mass spectrometers (MS) and differential mobility spectrometers (DMS) in our experimental work. Some of the instruments in our lab are home-built, some are commercially available, and some are commercial instruments that we have modified to fit our needs. For example, we have modified a commercially available hybrid DMS/MS instrument to couple with an external laser system so that we can study ultraviolet photodissociation (UVPD).

Key intruments in the Hopkins laboratory include:

• SCIEX Selexion DMS / 5500 triple quadrupole mass spectrometer
• SCIEX Selexion DMS / 5500 triple quadrupole mass spectrometer (customized for laser experiments)
• SCIEX 5600 triple quadrupole mass spectrometer
• Home-built velocity map imaging mass spectrometer
• Home-built parallel flow trapped ion mobility spectrometer (under development)
• Continuum Minilite II Nd:YAG laser (w/ frequency tripling)
• Continuum Powerlite Nd:YAG laser (w/ frequency tripling)
• Continuum Horizon OPO