Intense field Photo-ionization studies on carbon dioxide using a Velocity Map Imaging Spectrometer

Intense field Photo-ionization studies on carbon dioxide using

a Velocity Map Imaging Spectrometer.

Prashant Rawat, Kyung Sik Kang, Juyun Park, Jae-hwan Lee, Luu Tran Trung, and Chang Hee Nam

Coherent X-ray Research Center (CXRC) and Department of Physics, KAIST prashant@kaist.ac.kr

Interaction of molecules with intense laser fields is presently a topic of much interest in atomic and molecular physics. Depending on the laser intensity, many interesting processes such as ionization, dissociation, structural deformation and Coulomb explosion have been observed in molecules in these interactions. The intense laser field – atom/molecule interactions have in recent times also led to observation of many exotic phenomena such as high-harmonic generation (HHG), above-threshold ionization (ATI), laser-induced diffraction and double-ionization. The study of ionization dynamics of molecules irradiated by intense laser fields not only yields information about the inner structure of the molecules, but is also fundamental to understanding these exotic phenomena. Carbon dioxide (CO2) is an important constituent of earth’s atmosphere and that of other

inner planets. Also, being a tri-atomic, it is one of the few proto-type molecules being studied to understand the process of photo-ionization in complex molecules.

Intense field photo ionization studies on CO2 have been object of interest for quite sometime. Some

of the recent ones relevant to the present study were performed typically using the TOF-MS techniques(1-5). We performed these studies using the velocity map imaging (VMI) technique(6).

4 8 12 16 20 24 28 32 36 40 44 48 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 O2+ O3+

A

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itu

de

(a

rb

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Figure 1. TOF of carbon dioxide at peak laser field intensity of 3 x 1015_W/cm2_.

The VMI technique has advantages of, very good collection of charged particles, relatively good energy resolution and measurement of angular distribution in full 2Π symmetry. Thus, this technique is well suited for the type of studies being reported here. The target gas in our experiment was introduced into the interaction region in form of an effusive beam using a capillary tube. Intense femto-second pulses from Ti:Sapphire laser system, operating at 1 kHz with central wavelength lying around 816 nm, were used in a cross-beam configuration to photo ionize the target molecules. A position-sensitive detector (dual MCPs in Chevron mode) with a relatively large size (77 mm), a phosphor screen and a CCD camera were used in the detector system.

2 4 6 8 10 0 2000 4000 6000 8000 10000 12000

Photoelectron Energy (eV)

A m p li tu d e ( a rb . u n it s )

Figure 2. A reconstructed image from a raw image for O+

and the corresponding kinetic energy distribution for that ion.

Time of flight (TOF) mass spectra were obtained at different laser field intensities. Cation (CO2+)

and dication (CO22+) of the parent molecule, and fragment ions CO+, C+, C2+, O+, O2+ and O3+ were

observed in our experiment. The fragment ions were studied individually for their kinetic energy and angular distributions, and the results are being reported here.

References

1. J. Chen et al, Int. J. Mass Spect. 228, 81 (2003).

2. J. H. Sanderson et al, J. Phys. B: At. Mol. Opt. Phys. 31, L599 (1998). 3. P. Graham et al, J. Phys. B: At. Mol. Opt. Phys. 32, 5557 (1999). 4. S. Couris et al, J. Phys. B:At. Mol. Opt. Phys. 32, L439 (1999). 5. G. R. Kumar et al, J. Phys. B: At. Mol. Opt. Phys. 27, 2981 (1994). 6. E. T. J. B. Eppink and D. H. Parker, Rev. Sci. Instrum. 68, 3477 (1997).