Amounts of ethylene are insignificantly small - only 1 part over a billion of air, what requires special experimental methods. One is called photo-acoustics.
A beam of the infrared laser (CO2) excites vibrations of the ethylene molecule. Ethylene molecules collide with air molecules and transmit their excitation. If the laser is "chopped" with a frequency of a few kHz, a wave of faster moving molecules (a sound of this frequency) propagates in the gas – it is enough to use a microphone and we can hear it.
Never put bananas into the same basket with apples!
It has been known for a long time, that the presence of a box of apples makes ripen a whole hanger of bananas. It was not known why.
O n t h e T r a c k o f M o d e r n P h y s i c s
The sound is generated in the gas sample, if the radiative energy is absorbed, i.e. when at least one energy level in the searched molecule corresponds to the photon energy of the exciting
laser. If more exciting laser lines are used and more levels correspond, we get a clear fingerprint of the molecules. Moreover, this method is particularly suited for complex molecules, with many vibrational levels.
The CO
2laser produce many strong lines in the infrared range - this type of laser is the most frequently in the photoacoustic spectroscopy. Nowadays, also semiconductor laser are used.
Ethylene molecule has many absorption lines corresponding to emission lines of the CO
2laser.
10.2 10.3 10.4 10.5 10.6 10.7
0 20 40 60 80 100 120
10.69610.67510.65310.63210.61110.59110.57110.55110.53210.51310.494
10.28910.27410.24710.23310.2210.195
laser power (relative)
wavelenght (m)
Special thanks for schemes and explanations to Dr Andrea Boscetti CNR and IRST, Trento
0 4 8 12 16 20 24 28 32 36 40 44 48
0 4 8 12
C 2H 4 production (pmol g-1 h-1 )
time (h) 100%
RH
50%
RH
E
Orchid flowers start to emit ethylene just after being pollinated. In a dry atmosphere this process restarts.
Effect of delayed desiccation on the ethylene production of emasculated Cymbidium flowers. The ethylene production of non-emasculated flowers was low (~0.5 pmol g-1 h-1) and showed no significant response to the changing RH). E=time of emasculation.
from: E.J. Woltering and F. Harren, J. Exp. Botany, 40 (1989) 907, with permission
An adrenaline for plants, a gaseous hormone, is ethylene (C
2H
4). Apple trees emit ethylene, when fruits are mellow, orchids - when flowers bloom (and stop emitting when
they are emasculated), grapes - when a leaf is broken.
How do we know this? Thanks to lasers!
So, when photoacoustics will be available at home, after coming from work we will ask:
- How are You today Miss Orchidea?
A clue of the photoacoustic spectroscopy stays in the fact, that low-energy (i.e. long wavelength, i.e.
infrared excitations) have longer lifetimes. For this reason, the decay from upper levels undergoes through collisions with other molecules, rather than through a direct radiative de-excitation. A sound is created.
Spectral distributon of CO2 laser
Ethylene molecule and its infrared absorption spectrum around the 10 μm wavelength
http://webbook.nist.gov/chemistry
http://vpl.ipac.caltech.edu/spectra/c2h4.htm
