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1
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
How are you, Miss Orchidea?
An adrenaline for plants, a gaseous hormone, is
ethylene (C2H4). 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!
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.
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.
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.
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). Etime of
emasculation. from E.J. Woltering and F.
Harren, J. Exp. Botany, 40 (1989) 907, with
permission
Special thanks for schemes and explanations to
Dr Andrea Boscetti CNR and IRST, Trento
Spectral distributon of CO2 laser
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 CO2
laser 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 CO2
laser.
So, when photoacoustics will be available at
home, after coming from work we will ask - How
are You today Miss Orchidea?
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