Ion-dip spectroscopy

Vj = 1 <— v" = 1 transition will be at a different energy than the Vj = 0 <— v" = 0. We use this fact to measure the vibrational spectrum of V (OCO) in a depletion experiment (Fig. 12a). A visible laser is set to the Vj = 0 Vj = 0 transition at 15,801 cm producing fragment ions. A tunable IR laser fires before the visible laser. Absorption of IR photons removes population from the ground state, which is observed as a decrease in the fragment ion signal. This technique is a variation of ion-dip spectroscopy, in which ions produced by 1 + 1 REMPI are monitored as an IR laser is tuned. Ion-dip spectroscopy has been used by several groups to study vibrations of neutral clusters and biomolecules [157-162]. 

III. Laser-Driven Population Dynamics and Coherent Ion Dip Spectroscopy... 

III. LASER-DRIVEN POPULATION DYNAMICS AND COHERENT ION DIP SPECTROSCOPY... 

Coherent ion dip spectroscopy has been shown to be a versatile tool for the investigation of high-lying intramolecular vibrations in the ground state of molecules and of intermolecular vibrations of van der Waals complexes. 

Fig. 4. Ion dip spectroscopy schematic. R2PI is used to ionize and mass-select the cluster, Mx of interest. A second laser pulse, prior or coincident to ionization, excites an intermediate level (Sm), which depletes the ground state and reduces the ion signal of...

The use of laser ionization methods offered a more subtle and gentle way to ionize neutral clusters, which could then be mass-analyzed using time-of-flight (TOP) methods. Castleman first introduced these methods to clusters (named ion-dip spectroscopy) and applied to it electronic transitions. The method is depicted in Fig. 4. 

The downward transitions can be probed by the change in transmission of the probe wave by OODR-polarization spectroscopy or by ion-dip spectroscopy (Sect. 5.5.1). 

If the absorption of the probe laser is monitored via one- or two-photon ionization of the upper probe transition (Fig. 5.41b), the ion signal shows a dip when the tunable pump beam burns a hole into the common lower level population Ni (ion-dip spectroscopy). This opens another way to detect stimulated emission pumping. Now three lasers are employed (Fig. 5.41b). The first laser L is kept on a transition /) k). The molecules in the excited level k) are ionized by a second laser L2. In favorable cases photons from h may also be used for ionization, thus making... 

Fig. 5.41 Ion-dip spectroscopy. The ion dips are monitored by the depletion of the lower level /> (a) on the upper level k) by stimulated emission pumping (b)...

Snoek L, Robertson E, Kroemer R, Simons J (2000) Conformational landscapes in amino acids infrared and ultraviolet ion-dip spectroscopy of phenylalanine in the gas phase. Chem. Phys. Lett. 321 49... 

Keywords Biomolecules Conformation selective Free electron laser FTICR Ion trap IRMPD IR-UV ion dip spectroscopy Laser desorption Molecular beam OPO Tandem mass spectrometry... 

The types of action spectroscopy presented in this book include IR-UV ion dip spectroscopy for neutral species and Infrared Multiple Photon Dissociation... 

Fig. 2 Schematic representation of (a) a one-colour two-photon REMPI schane (b) the modus operand of IR-UV ion-dip spectroscopy (c) IR probing scheme for the excited state using UV— IR-UV spectroscopy (d) a general scheme for stimulated emission pumping for conformer A and B...

Fig. 3 a) mass selected one-color REMPI spectrum, b) IR-UV ion dip spectra, and c) IR-UV hole-bum spectra of the capped dlpeptide PheCys adapted from 8] The REMPI spectmm shows the vibrational progression of the electronic excited state of multiple conformations. The contributions of these conformers are visualized using the conformer selective IR-UV hole-burning method. The structure of the individual conformers can be probed by IR-UV ion-dip spectroscopy, yielding conformer specific ground-state IR spectra. 

Structure. As for ground state IR-UV ion dip spectroscopy, the depletion in the electronically excited state caused by IR absorption can also be probed via LIF. LIF detection should be equally applicable to ground state and excited states, but has more limited application as strongly fluorescent molecules are required [102, 104, 105]. 

In conclusion, laser desorption in combination with molecular beam high-resolution spectroscopic methods has enabled the structural characterization of gas-phase biomolecules of significant size. Moreover, IR-UV ion dip spectroscopy offers information about the number and types of conformers present. Applications to diverse classes of biomolecular systems such as peptides, carbohydrates and nucleobases, are described in the various chapters of this book. 

Stanley RJ, Castleman AW (1991) Cluster ion dip spectroscopy of hydrogen-bonded phenol (H20)n clusters, n = 0-4. J Chem Phys 94(12) 7744-7756... 

Robertson EG, Hockiidge MR, Jelfs PD, Simrais JP (2000) IR-UV ion-dip spectroscopy of N-benzylformamide clusters stepwise hydration of a model peptide. J Phys Chem A 104 11714... 

Snoek LC, Kroemer RT, Hockridge MR, Simons JP (2001) Conformational landscapes of aromatic amino acids in the gas phase infrared and ultraviolet ion dip spectroscopy of tryptophan. Phys Chem Chem Phys 3 1819... 

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