Vibrational chirp in the dynamic Stokes-shift process due to ultrafast geometrical relaxation in a polydiacetylene
Abstract
Relaxation kinetics of photoexcitation in a blue-phase polydiacetylene has been investigated by difference absorption spectroscopy with extremely high time- and energy-resolution using broadband sub- 5-fs optical pulses. One-fs-step tracking of an induced emission peak has enabled us to separate the vibrational dynamics during geometrical relaxation and internal conversion into modulations of transition-peak energy and transition probability. The peak energy shows redshift of about 60meV and the instantaneous frequency of the “ CC ” stretching mode is positively “chirped” starting from 1600±100cm-1 (21±2fs) and ending in 2100±100cm-1 (16±1fs) . This positive chirp indicates a huge dynamic structural change from (CRCCCR')n -like to (CRCCCR')n -like structured bonds in the polymer backbone during the internal conversion process within 100fs . This corresponds to the reduction of the bond length from 1.32±0.04Å to 1.19±0.04Å by about 0.13Å , which was estimated from the bond lengths in literatures determined by the x-ray analysis for several polydiacetylenes.
- Publication:
-
Physical Review B
- Pub Date:
- October 2005
- DOI:
- 10.1103/PhysRevB.72.134302
- Bibcode:
- 2005PhRvB..72m4302Y
- Keywords:
-
- 78.66.Qn;
- 78.47.+p;
- 82.35.Ej;
- Polymers;
- organic compounds;
- Time-resolved optical spectroscopies and other ultrafast optical measurements in condensed matter;
- Nonlinear optics with polymers