物理学家首次形成新分子

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物理学家首次形成新分子

  
电子围绕原子核旋转

              
科学家终于制造出了以前仅在理论中存在的分子--里德伯分子。

里德伯分子是两个原子通过极微弱、易变的化学键形成的。
  据《自然》杂志报道，这种新型的键只有在分子中的两个原子中一个原子的电子远离原子核的时候才能形成。

  该发现为主要的量子理论提供了支持。量子理论由诺贝尔奖物理学家恩里科·费米（Enrico Fermi）提出，是描述电子行为的理论。

  互动

  这种里德伯分子由两个铷原子构成，其中一个是里德伯原子，另外一个是"正常的"原子。

  原子中的电子围绕原子核运动，不同电子运动轨迹按距离原子核的距离由近到远形成不同的层次。

  里德伯原子很特殊，因为它最外层的电子层只有一个电子。

  早在1934年费米曾经预言说，如果另外一个原子"找到"这个独自游离的电子，二者可能产生互动。

  最早预言可能存在里德伯分子的理论物理学者，科罗拉多大学的克里斯·格林认为，恩里科·费米决不会想到能够形成这样的分子。

  他说，物理学家在70年代和80年代意识到，一个里德伯原子和一个基态（常态）原子之间有可能形成某种力场。但只有具备现在如此低温的实验环境后才能实际形成这种分子。

  研究人员在人工制造的负273摄氏度低温环境中，使两个原子之间的距离达到100纳米(1纳米 = 百万分之一毫米)的"关键距离"。同时，用激光能量把原子激化到可以形成里德伯分子的程度。

science_2

  World first for strange molecule  
               
   By Victoria Gill                                                                           
                                             Science reporter, BBC News                                                   

                        
         
                                                                               Electrons can be pictured as orbiting around a central nucleus
            
                                             A molecule that until now existed only in theory has finally been made.      
Known as a Rydberg molecule, it is formed through an elusive and extremely weak chemical bond between two atoms.
      The new type of bonding, reported in Nature, occurs because one of the two atoms in the molecule has an electron very far from its nucleus or centre.
      It reinforces fundamental quantum theories, developed by Nobel prize-winning physicist Enrico Fermi, about how electrons behave and interact.
      The Rydberg molecules in question were formed from two atoms of rubidium - one a Rydberg atom, and one a "normal" atom.
      The movement and position of electrons within an atom can be described as orbiting around a central nucleus - with each shell of orbiting electrons further from the centre.
                                                                                                                                                                                                                                                                                                                             It will be interesting to see what other fundamental physics we will be able to test with this approach         
   
               
                                                                                                     Helen Fielding, UCL
   
                                                                                                                                                    A Rydberg atom is special because it has one electron alone in an outermost orbit - very far, in atomic terms, from its nucleus.
      Back in 1934 Enrico Fermi predicted that if another atom were to "find" that lone, wandering electron, it might interact with it.
      "But Fermi never imagined that molecules could be formed," explained Chris Greene, the theoretical physicist from the University of Colorado who first predicted that Rydberg molecules could exist.
      "We recognised, in our work in the 1970s and 80s, the potential for a sort of forcefield between a Rydberg atom and a groundstate [or normal] atom.
      "It's only now that you can get systems so cold, that you can actually make them."
      Right place, right time      
      Unimaginably cold temperatures are needed to create the molecules, as Vera Bendkowsky from the University of Stuttgart who led the research explained.
      "The nuclei of the atoms have to be at the correct distance from each other for the electron fields to find each other and interact," she said.
      "We use an ultracold cloud of rubidium - as you cool it, the atoms in the gas move closer together."
                                                                                 The researchers excite an atom to the "Rydberg state" using a laser
            
                                             At temperatures very close to absolute zero - minus 273C - this "critical distance" of about 100nm (nanometres - 1nm = one millionth of a millimetre) between the atoms is reached.
      When one is a Rydberg atom, the two atoms form a Rydberg molecule. This 100nm gap is vast compared to ordinary molecules.
      "The Rydberg electron resembles a sheepdog that keeps its flock together by roaming speedily to the outermost periphery of the flock, and nudging back towards the centre any member that might begin to drift away," said Professor Greene.
      Pushing this electron out to its lonely periphery - and make a Rydberg atom - requires energy.
      "We excite the atoms to the Rydberg stage with a laser," explained Dr Bendkowsky.
      "If we have a gas at the critical density, with two atoms at the correct distance that are able to form the molecule, and we excite one to the Rydberg state, then we can form a molecule."
      This ultracold experiment is also ultra-fast - the longest lived Rydberg molecule survives for just 18 microseconds.
      But the fact that the molecules can be made and seen confirms long-held fundamental atomic theories.
      "This is a very exciting set of experiments," added Helen Fielding, a physical chemist from University College London.
      "It shows that this approach is feasible, and it will be interesting to see what other fundamental physics we'll be able to test with it."
      Prize-winning ideas      
      Professor Greene's prediction that Rydberg molecules could exist was inspired by another Nobel prize-winning piece of physics research.
                                                                                 The Bose-Einstein condensation was a new way of thinking about matter
            
                                             When, in 1924 the Indian physicist Satyendra Nath Bose sent some theoretical calculations about particles to Albert Einstein, Einstein made a prediction.
      He said that if a gas was cooled to a very low temperature, the atoms would all suddenly collapse into their "lowest possible energy state", so they would be almost frozen and behave in an identical and predictable way.
      In a sense this is analagous to when a gas suddenly condenses into drops of liquid.
      When scientists reached the goal of Bose-Einstein condensation, by cooling and trapping alkali atoms, Professor Greene realised that ultracold physics could be used to form molecules that simply would not exist in any other conditions.

http://news.bbc.co.uk/1/hi/sci/tech/8013343.stm
Observation of ultralong-range Rydberg molecules

Quantum chemistry: The little molecule that could

Rydberg molecule
Rydberg atom